Numbness or tingling is decreased sensations caused by altered sensory nerve function and this condition is common for children as well as for adults. It is very unusual that your doctor said that he doesn’t know what is causing this condition at your son.
Is this something that is happening very often or is it something new?
It is possible that your son is repeating some actions which can cause tingling in his upper thigh?
And have you notice how long does the tingling lasts because if it is just for a few minutes it is nothing serious but if it lasts for a few hours you should call your doctor as soon as possible and he can’t say that he doesn’t know what is the cause or to do nothing about it.
Let us know what has happened in the end.