Some people ride or lunge their horse with an overcheck. It can be a good idea if you use it for not a long time. It helps the horse to be in the right attitude and then to develop his muscles!
You can also do jumping "gymnatics" exercises with a line of obstacles. Driving reins are also interesting, you can work your horse differently and develop his body without being on his back!
Going for a ride and do downhill and uphill slopes is very good to muscle your horse abdominals and croup! And he will enjoy the fresh air and trees ;)
It is a daily work, you have to be patient! You can do a lot of exercises: transitions, different sizes of circles, voltes, counter-canter, counter-bending, bending, lateral movements. Those exercises will be effective only if your horse engages the hindlegs.