35 year old son says he has slight burning feeling in back of right calf & goes up into back thigh area, very minor, no swelling or reddness,going on over a month,could this be a clot? would it have gotten worse by now? could it be a nerve? worried?

There is little we can say with this information. Encourage your son to visit a clinic where a provider can take a history and perform a physical exam. Worrying about what it could be will pull you into a deepening vortex of anxiety and anguish for no good reason. It apparently doesn't concern him as much as it does you. At 35 years old, he'll get help if it's severe enough.