:yeah to all that up there :thumbIt's a normal part of the process. He may be going through a developmental phase that is preventing him from wanting to spend his focus and attention on that, or it may just be that he is making a little "mini-statement" (they can even at this age!). What you are doing, gently and consistently without forcing it or making it negative, is just fine. He will return to it. In the meantime, try to focus on the benefits of the communication part of EC as opposed to catching everything and him being peachy with it. Continue talking to him about it, speaking about being wet and dry, about going potty, etc. Gently. Don't stop completely. His resistance is probably totally unrelated to ECing, but you could lose the relationship if you just give up.
I noticed a major transition happened for him - the visit ended of what was likely a great time with lots of love and attention from grandparents. That is enough to throw off a child. Especially if he doesn't understand why they left, in his little kid brain. Did one of them do a lot of the ECing with him? Would it help to try ECing him while talking to them on Skype? Just for a while to transition him back to feeling secure again.
Also, little tricks might help if you haven't tried them yet: potties in different parts of the house - makes it fun. A new potty (something interesting to use, or a novelty). Maybe a "big child" toilet seat instead of the potty for a bit (my child liked this change and make it a part of independence development, deciding which one to use each time). Try going outside, under a tree or a bush (if you have the weather to do it!). Trying singing a favourite song or a new song just for ECing that he loves. Try telling a story or bringing a special item to potty time (just one, not a whole whack so it becomes play time).
Good luck! I think you'll look back later and say, boy, if I'd only known how short that phase would last!