how can i help my horse

my horse is 19 years old and has recently developed a limp in his left front leg, this only occurs when i ride him (i am not heavy by any means, i am a petite). i have been giving him MSM for the last few months, my question is, is there a certain excersise that he needs to be doing or is there a medicine that he needs to be taking?

At 19, it is most likely to be the onset of arthritis. Glucosamine is expensive, but worthwhile. I would contact your vet for advice to be sure he is not suffering from any spinous diseases (one of ours got put down with a spine disease and the lameness was onli light.)
   
If it is arthiritis, treat him with care and warm him up for longer before you exercise and reduce hard work for him such as jumping and galloping. You can still jump him, but just make his work less demanding on his ageing joints!