Are you looking to get him fit by hacking or would you be using a school environment ?
If hacking:
Best thing is to start first couple of weeks with walking ( say 20/30 mins) , building up time of walking (say 50/60 mins), then add in short periods of trot little by little extending distance. In week 3/4 ( depending how you feel the fitness is improving) add in short periods of controlled canter.
Start on flat ground if possible and add in hilly ground later on.