He hates family get togethers and will walk out of them without he is really possesive to he hates me seeing family also he's very insecure what shall I do:(

Why would you think he is autistic? Rather than just plain rude?

I know a lot of possesive, rude, insecure people, none of which are autistic?

Well my brother is autistic and loads of people I no think he is autistic

Who knows, has he ever been tested? Does his behavior bother HIM? Enough to be open for testing?

I no it bothers him a lot no he isn't been tested and I don't think he will

If you don't think he would get tested, then you may never know for sure. So, you should operate on the assumption that he is not autistic.
So, is he just rude? If his behavior bothers you then you need to talk to him about this. Communication is always key in a good relationship.