This has happened to my DS a lot, too, gglimpopo. My neighbour keeps asking "Is your DS going to the party this weekend?" She is trying to offer to take DS for me (along with her own boy), but there's no point, DS doesn't get invited! I find it very upsetting, not least because DS seems (to me) well-enough liked by his classmates. But if you aren't on very chummy friends with the mums, they don't invite your kids...
I am going to throw an entire year-group party for DS when he turns 6. May as well, may be the only party he goes to all year. Can you plan a special day out for your DS?