Oh Boy I hate Mystery Lamenesses. He could have an abcess waiting to blow, a pulled muscle, and edema.
I think the earlier you start them on joint supplements (and ourselves) the better as part of a preventative regime. Carlos is only 11 and I started seriously shopping around for "the best". Unfortunately the best is always expensive.
What I love about the smartpak website is it lets you compare the ingredients of supplements side by side. That will be your best tool in figuring out what to give your aging boy. MSM is good but don't forget about Glucosamine, Hyaluronic Acid and Condroitin.
Gordon Wright once told a student to take up swimming because he would never be a good rider, that rider was George Morris.