well, lets put it this way im 14 (almost 15) and very responsible. i have 2 boas and they are imo, the easiest animals to take care of. im not telling you to get a boa, because they could be a handful for someone with no snake experience, but if you want to research them a little bit you may find that you like them better than say a corn or ball. corns and balls are probably the most common and appropriate first snakes as they stay relatively small and are very easy to care for. it really depends on your (or your son's) personal preference. do you want a slender snake that will be active, or do you want a somewhat girthy snake that will not move very much? if you are looking for a slender snake then you should go with a corn and if you are looking for a girthy snake you should go with a ball.
feeding a snake frozen/thawed for is, believe it or not, actually much safer for the snake. rats can have very sharp claws and teeth which could easily injure your snake. therefore most people recommend that you feed f/t food.
both of the snakes i have mentioned are extremely good first snakes because of their ease of care and docile nature. i would recommend that you buy the snake from a local reptile show where you can handle it and ask the breeder any questions you may have. if you can at all avoid it, i dont recommend buying from pet stores because the snakes are usually not very healthy and are EXTREMELY overpriced. good luck and let us know what you decide to go with