I think it's very unlikely there were three class parties and he was the only one excluded from all of them, unless there is a list which for some reason your dc is the only one not on.
I've had 3 dc go through school.
Dd1 seemed to get invited to everyone's party. Boys, girls, smaller, huge, even ones in the other forms. When parties started getting smaller, they reduced down to just her 2 best friends, and occasional others.
Dd2 hardly got invited to any in year R. I can remember one time her saying everyone was invited except her, and, as she had tendency to exaggerate, I took it with a pinch of salt until I saw the mum with a list, ticking off replies, and yes, dd2 was correct, she was one of two not invited.
At the start of year 1, one of the parents came to me and apologised. What had happened was she had a very similar name to her best friend (think Emily/Emma) and the form had just called them both by her best friend's name. Her dd had given the invitation to dd2, who had said that wasn't for her and given it to her best friend. Then someone else overheard and said "that explains why dd2 hadn't come to their party and her best friend (who they hadn't been expecting) had"! She'd been thinking how rude we both were. 🤣 They obviously spread the word and I had a number of apologies for leaving her out, albeit accidentally.
So we made an effort of making sure everyone knew her name and she got a fair number of invitations, although not as many as dd1.
But also when parties started getting smaller, dd2's number didn't reduce at all, because she had a larger group of friends, rather than 2 best friends.
Ds is autistic (although we didn't know until later) and actually he didn't enjoy parties after the first half hour because he'd become overwhelmed, so I used to pick parties that I knew the parents would understand, and let them know I'd be bringing him a bit late, and that I'd stay to remove him so he could calm down if necessary. Any I didn't feel I knew the parents well enough we refused. He didn't get many invites though.
However ds is now 16yo, and gets far more invites than the girls did at that age. I don't think their groups did parties at all at secondary. Ds has more than he ever did. Although they arrange it among themselves and don't give presents normally, and are rather liable to turn up and rather than say "happy birthday" give an insult... but they seem to be happy.