A.V.
I think at 6 mo. DD was looking at us funny, but I would start with some simple signs (like milk or drink) and see what he picks up. It really helped my DD to be able to say she was tired or hungry or thirsty when she was pre-verbal and even though she's very vocal now, we still use signs sometimes to clarify (toddler language is hard to decipher sometimes). AND my friend's daughter is newly diagnosed with hearing loss, so we are all learning more signs to help communicate with her so the signs DD already knows are coming in handy. My DH learned sign language in college and we never used "baby" signs. We taught DD ASL to the best of her ability so that we wouldn't have to correct her later.
I remember when I met DH I was impressed at his use of sign with the big kids. He could tell them to "sit, be quiet" or "later" at the table or across a room and not interrupt a dinner or other event. And it was sweet when we were talking with a deaf neighbor and DD rolled out "thank you" for him (she was about a year old).
You may also find that he uses signs his own way. It's the way kids use all language. DD would ask for "banana" (the sign) and cheese. I realized she meant STRING cheese. Well, you peel it, right? :) We still call it banana cheese.