What a shame. Alas, I have found it can be hard to maintain friendships with non-homeschoolers sometimes, simply because they aren't available for social activities during the day and they are so busy with their own schedules. We tend to stay up late and are perfectly happy to go out late on weeknights or invite friends over until 11pm on Wednesday, but friends with kids in school aren't able to do that at all.
I suggest it might be time for you to try not to let it hurt you, by distracting yourself. Join as many homeschool groups as you can, and become extremely active in them. Once you know a few parents with children in the general age range of your son, invite them over for a small get-together, or even a weekend luncheon (dads included). Do anything you can to try to cultivate extra special friendships for your son but don't exclude his old friends. All you can do is invite them and see what happens.
I wouldn't explain too much of it to my 8 year old, for one, because you don't necessarily actually know the real true reasons for anything that is happening. You can keep it much more vague, by saying things like, "I'm not sure, sweetie, perhaps they are out of town this weekend" or "I don't know, perhaps they are just very busy with school. Unfortunately, they don't have as much time as we do now, do they?"