No, fighting Soviets does not make you a Nazi, joining the Nazis makes you a Nazi.
We can discuss the nuances of “voluntarily” joining a unit of an occupying army, but the fact is he joined, and I’ve seen about as much evidence of his regret towards that as I have of specific atrocities that he or his unit committed. So, he most definitely is a Nazi, but we don’t know whether or not he’s done anything particularly evil.