Not sure what is going on.

I'm not sure what to think. My son is four, he mumbles all the times won't open his mouth when he talks, twitches when he's upset or angry, will completly block out what your saying for no reason at all. Some people have suggested he's a bit autistic but I'm not sure what to think. He's a really smart kid but he just seems a little "different" from birth my mom had always said there's something "special" about him to this day she calls him "special" I really don't know alot about autism so if anyone could provide some insight I would greatly appreciate it. Thanks in advance.