Hey lisastpaul,
I think if you've ruled out all the obvious things like money, time, distance, religious objection, long standing feuds with other parents, car breakdowns... . then you might have more clear lines into what is going on.
I have 3 step and 3 bio kids. My H is residential parent for his S and they have always lived with us majority time. My 3SS 8, 10, 15 are all different tempraments and social abilities. My SS8 is pretty popular and gets invited to a fair amount of bday parties. His older bros are both socially awkward and both are on the edge of the autism spectrum and so they rarely get invited to do things.
When BPD bio Mom lived in the area (she now lives 1000 miles away) SS8 missed a number of parties because his Mom lived about 40 minutes away and was unable to get him to the parties due to her low functioning, lack of caring, lack of funds, feeling awkward about parties... . etc etc. We never pushed it because to have her drive up here for a 2 hour party in the middle of her visitation would be such a hassle for all of us. Just getting her to take the kids at all was a struggle most weekends.
My BPDexH also lived too far away to come back for bday parties and also did not like my DD doing activities that in any way cut into his time.
I would speculate your exBPDH is concerned mostly about himself.
Either he does not like parties or does not care.
It could also be that your S8 is not pushing it and so BPD Dad does not feel any pressure from him.
When my kids have had to miss bday parties and when other kids have had to miss mine-- sometimes we get a little mid week get together going and I pick up a gift and the bday kid and we have a play date and dinner and celebrate that way. It lets the mom know you and your S do care.
In the future, I guess you could pick your battles with exH. Talk to your S about each party and try to figure out which ones are the ones he actually cares about.
Then try using the S.E.T. tools to communicate with your ex on your S behalf.
TOOLS: S.E.T. - Support, Empathy and TruthSomething like, "Hey exH, I know parties are a pain to drive to... . but you know, these days you can just drop off and pick up, you don't have to talk to the other parents. S8 is missing out on Laser Tag, Swim, Karate parties and I'd really like to make this work for him. How can we do this for s8?"
If that doesn't work, then there is always mediation or legal action but i don't know how much of this is worth fighting for.
Talking through some of this with your S8 will help. Try to see where he is at with soccer and these other kids. He may be outgrowing the kids or have other interests or Dad could be being more controlling that you know or S8 may just be more into minecraft and basketball than soccer and want to hang with other kids... .
As for what to tell the other moms, just say Dad keeps him so busy on the weekends he has S8. I really have no control over his time with Dad but I'd love to have a playdate with your S.
hope this helps. Again, welcome back... . I hope to see you around here more!