What Does Pain, Tenderness And Swelling In Joint Of Wrist Suggest?

My 40 year old son is experiencing joint pain. It began in his wrist and swelled and was tender to the touch. It moved to his knee second - to the point he could not walk. Now it is in his shoulder. If I merely touch it - he winces. He has been to the 3 different doctors and they say nothing is wrong with him. One id prescribe an anti-inflamotory pill which has helped some.