Why is my horse lame?

I have a 17.1hh Knabstrupper gelding 20years old & he has been lame now for 3 months now... He is stiff& when he goes into trot, hes short. We got a back man out & he said hes jarred his legs from the ground being so hard due to lack of rain, however, one day hes getting better and progressing and the next day hes worse.. Whats wrong with him???