Could too much vitamin a be causing knee pain in 15 year old boy?

Unlikely: Knee pain in a 15 year old boy can be due to many causes: history of trauma (excess running, growing pains etc), inflammation (ie, juvenile rheumatoid arthritis), infection (usually obvious). Vitamins have not been reported to cause knee pain.

Probably not: Toxic levels of vitamin a can cause various bone problems but would not cause localized knee pain.