Suggest Remedy For Pain In Knee

My son, who is 4, had strep throat in September. 1 month later he woke up at night crying with leg/knee pain. He started having knee pain all day and it was very severe at times....he would limp on one leg. I brought him in and had a strep test done, xrays, blood work. All seemed normal so they put him on Ibupropen treatment for 5 days. He never had swelling, redness, heat or fever and was happy and playing normal. He seemed to get better for a few days and the leg pain returned, but not as severe....my pediatrician told me try physical therapy to strenghten him knees. I was not happy with that answer so I got a second opinion. This doctor did a thorough exam and more extensive bloodwork. All was normal except one test (not sure of name) which lead him to believe he has some type of post-strep infection. The normal range is in the 300 s and his was in the 800 s. He didn t seemed too concerned and put him on a stronger antibiotic for 10 days. My son was great for the 10 days and 2 days after the medicine was done. This morning he said he couldn t straighten his leg and that the knee caps hurt when he pushed on them. I am at a dead end and really need some advice. Thanks, Sheri