You need to ride a lot A build up his butt and to build his neck you have to get him on the bit
Hill work is great for building topline and over the rump, if you want strong neck muscles school him and get him to stretch his neck down so he works through it.
Put him in a paddock with hills or an incline - simple way way to build muscle. Maybe also talk to your local feed store about different feeds or supplements you could use. Also when you're working him, do exercises to build his top line
Riding out! Climbing and galloping have to be one of the best medicine! ☺