I break lots of Gypsy cobs for ride/drive. I would say they are up to gentle driving at 3. Gentle riding at 4-5. I wouldn't dream of putting one in to hard work till they were 5-6. At that age you work on their body, before then you are working on their minds. Keep everything short and sweet, no reason why he shouldn't be backed when it turns 4, but go easy. Hammer them down the road and their joints will suffer. Heavy breeds mature later than finer horses. They don't stop growing till 6/7.