Bicycles are a very personal item. If you know nothing about bicycles then you should NOT be buying used. You might very well be throwing your money away.
Just give you son the cash, suggest it is for a bicycle and let him make the decision. If he has not ridden before, let him do the homework and get idea of what he wants to ride. Then take him to a couple of bikes shops and let him do a test ride.
IMHO, I would not buy any bike with a suspension (shocks), unless I sure I would be using it for off road / off bike path use. You see so many people with these tanks on a bike path. There really is no need to carry that extra weight and the shocks are just one more thing that can break.