Hi Diana, you can try Audevard products for joints, as well as OrJoint from OrVet which does an amazing work as a daily supplement. As a daily routine, don't hesitate to relive your horse as much as you can with good local treatments after an effort (ice, clay…). You also have to be very vigilant about the grounds on which you will train him (if their are too hard, the shock will be much less absorbed and will do very bad on his joints). Take your time to warm him up, and if you can, you should try to have him injected once in a while to make sure to avoid inflammations and give him more comfort (I do that every year before show season for my elder horses) :)
Injecting the horses every 6 months in the joints extends the life span of the horse's joints significantly. I would also look into the back on track products and consider and omega oil supplement such at the one from Lozana Sante.