firstly you want duel disc. its a lot easier to maintain and (not in my opinion, but neibourgh, nearby people's opinion) sounds better. discs also work best in all conditions.
secondly, seens he is ten he problerbly will not be very strong compared to adults/teenagers so i would try find a reasnoble light bike for him to make it easier. id guess 8-9 kg give or take .5kg
thirdly it will be easier for him on a higher, and shorter bike. 990 or shorter and 80 or higher.
it all depends on budget really to be honest, and the difference are not very large at all.