You can try looking into supplements or changing his feeding routine to aid with the process. the best thing to do is to change his exercise routine. To build muscle in his hindquarters, try lots of hill work. A few times a week start walking him up small hills and gradually increase the size of them as time goes on. Eventually, he will be able to handle more and yo can start trotting them. The best thing to do to build neck muscle is to do tons and tons of transitions. These really help with focusing on how he carries himself. Collecting and extending, trot to canter and back down again. remember to frequently stretch him out to avoid him getting stiff and sore. Also, try quine massage/massage blankets after riding to help the muscles restore and rebuild.