If they are taking him to the doctor for checkups, hopefully the doctor will catch something and act accordingly. Some of these "red flags" might be indicators, but kids mature in different ways. Aggression isn't a sign of autism - it's generally a sign of frustration. If he has communication delays, THAT would be frustrating, and may be a source of aggression. Also, avoid the "in his own world" concept - if autism is the issue, he may be experiencing the world in a different way because sensory processing is often very affected (things are too loud, too bright, too everything) - so what non-autistic people call "in their own world" is "trying to avoid sensory overload".
All you can do is be a good aunt. Be there for him when he's interacting. Please visit autistikids.com - lots of info there, and blogs by autistic people of various ages. If it is indeed autism, the more knowledge you have, the more supportive you can be.
Perhaps suggest "baby sign language" if he's not yet talking - there are books in the library, websites, etc., and you could even do this when you're with him. It might not help much if it's just coming from you, but if the parents start to use it and it helps him communicate, his aggression may subside.
I hope everything works out ok for him.