What I do is lunge my horse with a roller on his back and have the side reins long so he can lower is head and outwards I also feed him on a high protein diet with amigo oils so he develops his topline and also has a nice shiny coat.i hope to do IPS with him and also under the saddle flatwork do the same long and low this will also develop your position don't always have him in a frame of you do he will get stiff and develop the wrong muscles even after a ride cool him down by trotting in a circle and he should move from behind and working correctly to lower his head and neck it all comes from the backend. Hope this helps🐴👍🇮🇪
I have the same problem with my horse and it does take time however u see and feet the results.
If you have the possibility go hacking. Lots of hillwork are the easiest way to build a great topline without forcing anything. As long as you maintain a calm and steady connection to it's mouth, your horse will carry it's neck use it's back muscles and develop strength from behind. Try riding hills backwards up and down or different side walks to work all the muscles from hindend to the neck. And it's also relaxing for both of you not to ride indoors all the time.
If you don't have the possibility, try to ride transition's, transition's, transition's to playfully work those muscles. And maybe some little cavaletties to keep it interesting.
Hope this helps.
It takes two years to fully develop a top line. It’ll come with time. Stay committed, and make lots of short term goals. You’re probably making lots of progress if he’s working in a good frame three times a week
Hill work is excellent, also teaching your horse to drop long and low and keeping the paces forward. Make sure that your horse is using his bum and back and just because his head is tucked in doesn’t mean he’s using the rest of his body properly :)
Lunging over raised pops in a good frame helps my horses maybe you should try it if you have the equipment ☺️