My horse has a very hollow back, how can I build this up?

If there really aren't any hills, lunge. The circling really helps with building muscle. You also might try putting him on a supplement. I've always had the opinion that trot poles really help, but I don't have anything to back that up. If the creek is deep enough, swimming is also a big muscle builder. Do a lot of transitions, and try to keep him in a frame and bending. Good luck!