Just take it slow! Start with just walkig him under saddle for 15-20 minutes a day, working on bending and flexing, maybe some slight inclines or ground poles. After a fewdays, add in 5-10 minutes of light trot work for a week or three, keeping your rides 20-30 minutes. Your warm up and cool down should be lengthy, so you don't tax his joints or endurance.

After that, just take it slow, and watch his reactions.