Sims are great, they teach you a lot and will speed up the learning curve a ton. If money is tight, there are free sims that will do the same thing and you can get the plane too. You can find used to help you with the price of stuff and get into it a lot cheaper. Depends on your goals.

Personally, if it were me, I would say get a good plane (.40 size) and a good radio with a buddy box and a good engine and join a club, get a good instructor and fly. The sim is good, but your son will tire of it very quickly if he really wants the plane. I use mine a ton to learn new stuff for the heli, but hardly ever use it for fun any more.

Just my 2 cents.