Keeping the lines of communication open!

My autistic18 year old has never been much of a conversationalist,  but currently think we are unable to speak to each other at all. Even if I ask questions about favourite topics (Minecraft and YouTube) I get a grunt or get shut down. He's 18, I get that. I struggle to know what is the behaviour of an autistic person and what is the behaviour of an obnoxious 18 year old. Any tips on how to keep the lines of communication open? Thank you.