First of all, this is just the coolest thing that you want to do this for him. He's a lucky guy! ..and I DO NOT want to discourage you from getting him a bass, but...
Buying a bass is usually a very personal thing...it is almost impossible to pick out the correct instrument for another person unless they have played it before. There are so many things to consider and many personal-preference issues.
Maybe there's a way you can get him a bass but still allow him to select it. Either by visiting music stores with him and find out what he truly likes or by giving him a gift certificate to his favorite music store to select it later himself. Some music stores have flexible return policies so you could get him something and if he doesn't like it, you could exchange it with no loss of value.
Think about how much you are able to spend, because that will be the first determining factor. Basses can range from $100 to $10k+. You can find good instruments in the $200-$800 range - but it depends on his expectations. Find out how much he has spent in the past.
Your best resource might be his band mates or people he has played with
Good luck and if you decide he's not worthy - give me a call!!!!
