Ha! I am in the same boat. My one gelding has no butt or a bad butt. What I have done is increase my horses feed. He was a bit thinner than I would like anyway, not skinny but not quite perfect weight. I figured he could start to throw some of the extra calories into muscle. He is also on turn out 24/7. Since the feed increase he is running around in the pasture a lot more. Which I regard as part of his independent exercise program.
I also started to trot him over long distances. We have been going on maybe 1 to 2 mile trots. I am lucky because I have an area I can do long trots without having to turn. I am also becoming aware of how long this process is. It's like putting weight on a horse, you don't really notice a dramatic change. Keep at it, if you have questions it also might be worth it to talk to your vet. They might have a better idea about what feed is appropriate.