Best thing is to ask what he likes and doesn't like. I used to hate when people finished my sentences when I was younger, now, 99% of the time I don't care at all, unless I need to make a clear statement. Everyone acceptances are a like different, but most of use have had similar experiences. End of the day , talk to him and I'm sure just having you around and willing to wait as along as needed for him to finish his words will mean the world to him too.