I would go with a carpet python, personally. Irian Jayas are good, because they are pretty small (4-5.5 feet) but not tiny. And they are pretty tame, even babies sometimes. Coastals are awesome too, but get a bit bigger (7-8 foot average). You can go with a jungle as well. Either way. just try and make sure that its tame, well established, and feeding well on RATS.
This might be a little lame, but since he's only 14 you might want to go through different snake choices with him and tell him to choose the favourite one. Then tell him to research it and have him take a test and he can only have a snake if he passes
Just an idea!