It might be because he can't see what you are doing when you get to his back legs? I had a horse that was terrified of being washed on his legs, I eventually got him round by doing a tiny bit each day, starting with the water on his hooves, then moving a couple of inches up the leg, then rewarding and stopping.
The next day I'd do the same again, gradually moving up his legs. Each time I would take it very slowly and make sure he was aware of what I was going to do.
If you think he is really too scared, or you are going to be in danger, I would stick to a bucket and sponge.
Patience, just hose his hooves or even just near his feet untill he's comfortable with that an work your way up a bit at a time. Hosing back legs tends to splash more because your hanging on to the head and hosing from a distance. They are also more sesitive on the inside of their thighs and under the stomach and may not like being splashed there.
If it turns out he's really scared of the hose sponge his legs down but have a cut of piece of hose with you. eventually (hopefully) he will become used to it.
Is he happy about you handling his hind legs in general or does that bother him too?
Maybe spend as much of your grooming time as you can just fiddling with his back legs, touching them with all sorts of things other than the brushes he is used to - just to let him feel different sensations on his legs and then take it from there.. Just a thought - does he let you touch his back legs with the hose if its turned off or does this still get a reaction from him?xx
All horses deserve, at least once in their lives, to be loved by a little girl