Here's my two cents: get him a bannana female and see how he does with it. Have him learn more about proper care and all that jazz. If he doesn't get bored with the BP in a year or two then he can think about breeding. It also depends whether you're going be both interested in the hobby.
At 15, his interests might change very quickly so unless you want to be stuck taking care of all the snakes your son tries to breed, I suggest starting with one and going from there. You also have to think about what will happen if he leaves for college.
Depending on how much you want to spend, you can either get a glass or a plastic (PVC) terrariums. With PVC terrariums you're going to have an easier time with temperatures and humidity but the cost a bit more than a glass one.
I think a BP is a great gift and it's awesome you're learning about them as well. I'd hold on the breeding plans for now though.
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