You can pick up prepaid phones really cheap nowdays ($25-30ish). Still it is a bit of a strange present. I guess it comes down to wether you want him to have a mobile or not at his age. If not, you could always put it away until he is older. Or maybe negotiate some rules about when he is allowed to use it (ie, if he goes out with his friends, but not just all the time).

Pesonally, I would be annoyed if someone bought my kids a mobile, because that is a parental decision as to when a kid should be getting and using a mobile.