Your son would probably freak out and say he wanted just about any black snake he sees.
Give him pictures of all the different black snake options he has, then tell him their habitats, how he would have to set things up for it, the kind of care it will need, humidity, temps, size, etc. Then let him make an informed decision. At least you can then help him learn some reasoning skills instead of doing all the work and making the call yourself, or him just making the call based on looks.
And before getting the snake, write out a list of everything you'll need for it and go shopping for those items with your son and let him go through the list instead of you going through. The more responsibility and freedom to think you offer your child the better off he'll be in the long run (I go to college with people who are still mentally 13, and not in the joking cute way but in the not mentally developed couldnt function if they didnt live with their parents no jobs would keep them for more than the training period way).