Bless you, of course it hurts! I'm so sorry. Children with additional needs get a rough ride with the social events and it hurts to see our lovely kids being left out.
The unhappy reality is that people want an easy life. They invite the children they know, or the ones they think their child plays with. They don't consider the feelings of anyone not invited, and they often think in terms of ease and crowd control. They don't mean to be cruel, but it's painful nevertheless.
When your child is one of the more 'challenging,' has additional needs, or can be perceived as loud, 'a handful', awkward, impulsive, or anything like that, the invitations aren't there as much.
The good part at this age is your DS is unlikely to be aware he's been excluded from something. Later on, they notice, and helping them navigate that is important.
On a practical level, not everyone is invited to everything. Every child gets left out from something at some point, and no one "owes" us invitations. I found it easiest to view it that way, make our own arrangements (sometimes pr-emptively when someting was looming on the horizon) and not let us get too caught up in what other people were doing.