Does he go outside? If yes then I think a collar is a must, if he's strictly an indoor cat or going to be then I wouldn't get one. I don't have one on Maya but she is kept indoors at all times, and if we go outside she is on a leash.
Most collars that I have seen are adjustable. I remember my sister got collars for her cats that were outside cats, and they had this thing so they would easily open incase it gets caught in something. The collars usually dissapeared after 30 minutes of being outside, I have no idea how.
I know that Petsmart sells a big variety of them. If you do get him a collar I would probably go with the ones that open in case it gets caught in something, especially if he will go outside, for safety reasons. But that's just me.