OK. I live in Bangkok. In Bangkok my street bike (CBR 150) and my track bike 05 GSXR 1000 had standard shifting. When visiting the islands in the south I ride a moped (125cc.) The mopeds have GP type shifting without a clutch. I bought a track bike in the states (05 GSXR 1000) with GP shifting. I loved it. Changed my track bike in Bangkok to GP shifting as to be consistant. Loved it too. It just seemed to feel more natural.
I ride the small street bike everyday. The track we ride here doesn't have a lot of gear changes but I have downshifted by accident and upshifted by accident in what could be considered critical areas of the track. I decided to change back to normal shifting on the track bike because of that issue.
I sugest that if your son is riding on the street and he cannot change the street bike to GP shifting, keep with standard shifting. Especially if he doesn't have a lot of experience on a bike. For me it was easy to adjust to GP style, but if I ride another bike with standard on a regular basis, I have difficulty when returning to the track.
If not riding on the street I say choose one and stick with it. Otherwise he may have to devote unneccisary attention to shifting while on the track. As Keith says, we have a limited amount of attention to spend....
Have fun!
JP