Lots of stretch throughout your warm up especially! Lunging is very beneficial as well! Just make sure to lunge with a bridle so you're able to ask your horse to stretch to encourage them to work over their back!
i like doing ground poles and cavelettis
Another ground work exercise that helps is taking like your finger or nail and going up his stomach working from his shoulders to his butt. It should make him raise his back and curve all around!
Long and low up hills and over poles.
I would suggest doing a lot of long and low work. So, stretching, stretching will going up hill, and working on some different trot pole excersizes.