The advice you’re getting is all very good and perfectly accurate. I would however add one thing, contact your local Parent Information Network or Parent Information Center. If you’re in the USA most states have one, they are often called PIC or something to that extent. They can be a great resource for you and will oftentimes send a trained volunteer to your child’s IEP meetings with you to assure he gets proper services and they will be able to help you find services you may not have ever thought of.
I would also recommend that maybe your son might benefit from talking to the school social worker or school psychologist; because if he’s being socially cast aside or left-out he’s probably feeling it as well, and likely holding in some emotions he may not even be aware of. Having a safe place to talk that isn’t a parent can often be a big help and they can help him develop age-appropriate strategies for dealing with this and on how to become more included.