Maybe have a chiropractor who works with children check him out? Here's why I say this: My daughter, who is now 17, was playing with her older sister when she was 18 months and her sister was about 5. Sister was jumping off the couch in the living room onto a pillow. Little toddler follows, lands on the pillow, gets a funny look on her face (confused--not really hurt), and collapses on one side, falling down. Big sister laughs as toddler tries to stand up again, with toddler continuing to look confused. She never once cried or looked like she was in pain, but she could not put weight on her leg--it kept collapsing under her when she tried.
Since I had seen it happen, I knew we needed to get to the chiropractor--it was like a nerve was pinched or something, but not painful. But my insurance at the time required a primary care physician's referral and I didn't want to pay out of pocket for the chiropractor.
Her pediatrician looked her over and offered an Ace bandage and antibiotics! I told her my daughter was not sick and asked why the antibiotics--she said because it could develop into an infection or something. She would not refer to the chiropractor.
So I took my daughter to our chiropractor anyway, with a little girl who could not stand up in the waiting room without a tightly wrapped Ace bandage. After her treatment, as I was paying the bill, my little girl--with no Ace bandage anymore--was doing ballerina twirls and climbing all over the furniture!
I'm not sure your son has the same problem going on, but this experience can let you know that a) sometimes stuff happens structurally that requires readjusting a joint in order for it to function, and b) doctors don't always approach it from the correct perspective and can actually make things worse (antibiotics? Really?) by viewing it through their own lense.