One thing for sure that child has no joint disease
. Asfor leg pains , we have to see whether the child has a balanced diet,with regular 2-3 glasses of milk, legumes, fruits and calories.What about the feetare they normal contourd I mean hope he has no flat feet or knock knees. WHAT is the height and weight of the child?sometimes nutritional imbalances could account for muscle aches and pains especially if the child is hyperactive , doing too much running or playing. All this you will have to look into and cause may be detected.Mild raise in ESR
has little value unless some apparent symptoms of an disease is there.however ,to rule out muscular disease get serum CPK level done.
You may also consult neurologist/orthopedist and ask for electro-physiological studies of leg musculature -if any deficiency in power of muscles is there would be known
Give him multivitamins, calcium tabs and do massage
of leg muscles with olive oil (olemessa oil) and wet towel hot fomentation after he is back from play in the evenings.e