My Son Was Exercising, Doing Lunges And After His Workout

My son was exercising, doing lunges and after his workout became very weak in his legs. When I say weak, he fell, twice. THe second time was when he was standing straight up and his legs just would not support him and he fell to where his buttoks landed on his ankle. He thinks he may have hurt his knee as part of that. We will get his knee looked at but right now he is in bed and unable to stand. He says his right leg is gaining strength but his left leg is still weak and there is pain in his ankle and knee when he tries to stand. He has no pain when resting and no swelling in either leg or his left knee.