When my DS was 4 he had ten kids from nursery invited to his birthday party. My ds was not really verbal (very hard to understand what little speech he had at that time). We invited five kids that nursery told me he played best with and five to make up numbers and balance the girl and boy numbers. I have no idea who we excluded, no one on purpose. By the time he left nursery he had two very good friends he adored, despite the fact his language skills hadnt really caught up. My son is NT so I know it’s not the same problem but I wanted to reassure yu that the logic for inviting kids is often flawed -aside from very close or family friends, who is who at nursery is not really very well known by many parents.
Also my ds best friend (not at nursery) had only a family party as it’s a large family and his mum felt a nursery party too would be overwhelming (and same again when he was 5).
Have you organised play dates with nursery friends eg meet up at local soft play or playground?
Do you have the phone numbers of his best friends’ mums?
Did you invite the nursery gang to your DS 4th birthday?
A lot of this is going to be down to you to do some social engineering.
If ChildA is having a birthday and mum says, “you can invite 12 friends, who do you like playing with most?” Then if your ds has been having trouble with social interactions he won’t be top of ChildA’s mind, or he’ll be top of mind for the wrong reasons (ie childA says he doesn’t like him and they don’t play together).
So Instead of getting depressed about it, DO something. Create some social situations where you know your ds will be in his element - whether that’s running round the park, or a 1:1 play date with cars and cake decorating. And invite someone from nursery.
your dc is only age 4 you have plenty of time to help him find buddies. He will, in due course and with some help.