The seat is usually "slammed" and not used except for sitting while stopped. The bars can be tilted back slightly but the bike will still be too big if he intends on learning park/street tricks.
Unless your 5ft 11yo son is a heavyweight a full chromoly frame and dbl walled rims is not that important. Get him another bike that fits (even used) to learn on and keep the 20.75 for yourself (for now) so you can ride with him.....