I'm by no means an expert, but your little man sounds pretty normal to me!

You have the added worry of comparing your twins, which I can imagine is difficult when one is doing things (answering to name etc) that the other isn't, but all babies are different, and whilst I don't have any experience with autism, the signs that you've listed don't sound like autism alarm bells to me.

Besides, six months is extremely young to be diagnosing anything like that.

I'm sure your little man is perfectly fine and your babies are just a little different, thats what makes them individuals