Hello Tandum,
I take it your son has been riding a gs500 and was it restricted? Either way it should have had enough power to have learnt the basics.
I would vouch for upgrading. I feel a lot safer on my 1200 than I ever did on the 4 stroke 250 that I had. Nobody can keep up with you on the roads. Don't have to play bumper cars with the mental chip on their shoulder person at the end of the day who plays traffic boss. You just leave them all behind.
The only thing I would say is go for the lightest bike as it will be easier to manouvre with. Probably the only setback with my 1200 is it is a little too heavy.
And buy him a day at the racetrack so he can learn to leave his ego there. On the road ride to live to ride another day is a motto I like.
Cheerz
Simmo