It's wonderful that you're doing this for your son, however...
Maybe he should shop by himself? My experience with my wife, daughters, etc. has taught me that making a choice on someone's behalf and then having to sell that choice is twice as difficult as a decision needs to be
no matter how right or perfect I think it is
. If he has a firm grasp on his budget, an idea about what he could trade or sell is current CC for, and an idea of what he's looking for, he'll pick the perfect
available bike when it shows up. It pays to be patient when looking at used bikes for obvious reasons. If he makes a mistake, it's his and he'll learn from it. If he scores the perfect new-to-him bike, then it's his win!
OTOH, there are bike shops and stores near me that will take a used bike and offer a discount on a new one, helping you make the budget for a new bike that might otherwise be over budget. I'm fairly sure Trek accepts their old bikes back for trade only, but I could be wrong.