I understand your feelings!
My DSS, who's the same age as your son, lives with us and Mom lives maybe 30 hours away. So, while I'm not his mom, he's here in my home as though I am, the vast majority of the year. Like you, I'm an at-home mom and do all kinds of fun things with our other kids in the summers. But DSS spends 7 weeks with Mom, so he is gone for much of that. Plus, DH and I feel DSS's whole situation at our house (on a host of levels from supervision to diet and exercise to freedom from emotional abuse) is healthier and more appropriate than what Mom provides. So it's hard to think about him being there so long.
It's also natural to think, "If the NCP doesn't take advantage of the opportunities they already have, to be here, visit frequently, get involved in the kids' regular lives; then it's not fair that the NCP takes such a big chunk of that summer time which feels like it ought to be the CP's reward, for shouldering all those school-year responsibilities! It's just not fair!"
So I understand that my advice will be hard for you to hear.
1. If your ex exercises his 6 consecutive weeks in the summer, it will improve his connection to the kids. They will finally go beyond just "visiting" each other and feel, for a brief time, like they're living together - establishing routines and learning each other's quirks. They'll move past "best behavior" into sometimes being bored with each other, sometimes being short-tempered...but sometimes feeling a genuine sense of familial intimacy - which you don't feel if you're not visiting someone long enough to bother unpacking your suitcase.
2. Unless your ex is abusive (not as in, he speaks to the kids more harshly than you do, but abusive) - which people usually mention straight away, when they're upset about their kids spending time with their ex - then it's better for your kids to have close relationships with and spend some sort of significant time with both parents. Let's just assume your ex is a fundamentally worse person and worse parent than you are. Your kids still - inarguably - recognize that he's part of them and will grieve not having a close connection to him; or not spending much time with him while growing up...even if they never tell you that.
3. As kids of both sexes approach their teens, time with Dad becomes more important. (Which is not to say it's unimportant, when they're younger.) Identifying with their same-sex parent - and feeling important to him, worth his time - boosts boys' self-esteem. I'd argue it's even more important, for girls. An 11-y-o girl is on the verge of wanting boys' attention and affection...and NEEDS to know how much she brings to the table, in a relationship with a male, beyond sex. First off, girls tend to crave less attention from boys their own age, if they have their father's love and attention. Secondly, just by showing interest in and spending time with daughters, fathers raise the volume of a girl's inner voice, that says, "If the main thing a boy sees in me is the quickest path to get my clothes off, he's missing all the most important things about me, so he's not worth my attention." Mothers - even if they use all the best words in talking to their daughters - cannot fully accomplish what a father's attention can, in this regard. Even if your ex is not the ideal father, he still has a lot to offer your kids just by spending time with them.
4. So, if the main time it's possible for your ex to spend time with the kids is in the summers (due to the distance), then lengthening those visits is actually a good thing, for them. It's not a good thing for you. So the most loving, selfless thing you can do is to separate those two things in your mind, as you help them deal with the anxiety kids may have over any major change in routine - even when it's something positive. You have the choice to convey either message:
----- "I know how hard and upsetting this will be for you - being away from me; missing all our summer routines; only being able to hear my voice once a day. Let me find every way I can to reassure you that I'm always with you, in spirit, and that our separation will be over before you know it. Oh...and I'm sure it won't be too bad, with your Dad. I hope you have fun." OR
----- "It's important for you to spend time with your Dad. He loves you. I'm very lucky to have so much time with you. I never want to be away from you, but the reality of divorce is that you can only be with one parent at a time. And we're far from the only divorced family! It will be good for you and your Dad, to have some more time together. Yes, it's a change from how we've done things before. But life changes all the time. We roll with it and see what the changes will bring. We don't let ourselves get stuck in worry and fear, just because something is new and different. This will be good."
In the end, the kids will feel either good or bad about the summer arrangements based on how your ex handles his parenting time. You will affect their initial attitudes and what they feel they need to tell you, about the visit. If they sense that your self-esteem, as a mother, hinges on them feeling insecure and desperate to return to you; that you want to believe they only feel nurtured by you...then that's probably what they'll tell you. So if they actually have a good visit with their Dad, they'll feel guilty about it...then guilty toward their Dad, for telling you they didn't like visiting him.
So be careful. Mothers can say words that sound loving and comforting, but that really convey, "I don't want you to feel loved, comforted or secure unless you're with me." And mothers can do that unintentionally.
5. The best way for you to get through this - and convey to your kids that you're OK (so they can be OK, too) - is to find something constructive to do with your time, while they're gone. If you find work, great. If you don't, volunteer. Join a book club. Start an exercise routine. Repaint the house. Just don't sit around and miss them. There is no reason for you to feel guilty about enjoying your time away from your kids, if they are doing something they need to do, that can be important to their development - i.e., get to know their father.
6. It is OK that different parents have different standards, in their separate homes, about how much access kids should have to electronics. It is OK that kids talk to their other parent on a phone that belongs to the parent they're with. In our lifetimes, families used to have only one phone and only one answering system. It is even OK that your kids only talk to you once a day, while they're with their dad. That lets them check in, gives them a chance to tell you about their day and hear about yours. But it also means they're not going to be constantly interrupting their time with him, calling and texting you or playing games on their devices. It means when they're bored or irritated with their Dad, they'll have to resolve it somehow, not whip out their phone and complain to you. It's really OK.