The yellow tone is due to the high bilirubin levels that are caused by the premature breakdown of red blood cells. This is very common to see in thals and even many minors will have some yellow tinge to their skin. There is no reason this should affect activity level, so allow him to play as he wishes.
Taking a B-Complex supplement, vitamin E and magnesium may help the blood last a bit longer and reduce the early destruction of red cells, but I don't think it can be eliminated.