These are good student horns. Important attributes to assess it's value? In my opinion, I would say first playability. Does it play all the notes easily? If not, you could have a problem with leaks and the horn will need an adjustment and likely some pads replaced. Equally, or more importantly is intonation. Do all the notes play in tune? A good way to check intonation is with an electronic chromatic tuner, although this may not be an easy task for a beginning player. Probably least importantly is appearance. Does it look good. Of course, nobody wants a horn that looks like junk, full of dents, etc. Probably your best bet would be to take the horn to a reputable repair person and have the horn checked out by him(or her). That person can likely give you a good estimate of it's value as well as any repairs it may need. Good luck and tell you son not to give up, even when things get tough. He'll be happy one day that he stuck with it.