Stretching :) get a toy for them to push around on the ground all day
Walking and trotting up and down hills. Asking for him/her to lower their head as they go so they engage the back. Also lots of good transitions. Pole work and gymnastics help as well. Lounging with side riens. It takes time. You won't see results right away.
I had a horse that needed work on his topline and I did a lot of flat work while framed. I would do a lot of circles that helped with his balance and made him use the muscles in his neck. Also doing a lot of lateral flexion and doing carrot stretches :)
I recommend starting to learn how to get a horse into a frame or lunge then with side reins