If you have access to trails, use them! My show horses do most of their conditioning on trail. It's great for their minds and gives them a purpose. They can see WHY they need to leg yield around trees; WHY they need to engage their rear end and back to get up and down hills, and it keeps their minds fresh.
Start slowly and build up. Trotting builds more muscle than cantering, so do lots of trotting in intervals to build up his stamina.