I once had a mare in similar shape. I did the carrot stretches making sure to go side to side, chin to chest, etc. I also rode her up and down a lot of hills.
If you don’t have any hills where you live, try doing some cavaletti exercises with him. That is very strengthening, helps get his legs under him. Go online and lay out the poles on the ground at the recommended distances according to gait. Putting those poles on the ground will help him a great deal.