Working long and low to start with and gradually building up, making them stretch over their back and use the muscles. Possibly lunging with a training aid e.g pessoa or chambon too.
the best thing to build up a topline is consistent work that allows the horse to use himself/herself correctly. It is a slow process and I don't know of any feed that will make a horse develop a topline (same as there's no food I can eat to give me tight abs
).
Make sure that saddle fits to allow the horse to use his back properly. All work that allows him to travel straight (rather then crooked). Transitions, circles, leg yields, shoulder in, haunches in, serpentines, hacking out.
Baileys lo cal or stud balancer I have found have been very good for helping build topline, obviously the horse needs to be working correctly to build the correct muscles but presuming your horse is in good weight, I would reccommend the baileys balancers, they have put amazing condition on my horses.
Working long and low to start and using transitions, hills and poles to encourage them to step under. changing the size of a circle on the lunge combined with transitions in and out of the same gait really help them to work from behind.
Feedwise - I'd swear by winergy equilibrium - mine have never looked better and it's more natural than a mix or a cube.