He honestly was balding. Here is photo from 2011 Comic Con when he was there to promote Immortals. He very clearly was balding.
But in the last 2 months, on the Man Of Steel promo tour, he has looked nothing less than the most handsome man alive, and I say this as a straight fella. So I am sure something was done about it.
There is also this snippet from an interview from Immortals where he indicates as much.
He needs "a little product help" for the hair. ("I'm not going to lie," he says.) But the body that will fill the famous suit will be all his. "This," he says, gesturing to his chest, "takes hours. It's been two hours a day for the last eight months."
Now that he is an A lister with a major studio, he will obviously be taken care of so that he always looks his best but he definitely has had something done.
But this is Hollywood folks, even if he went completely bald, Hollywood will make sure that he always looks as handsome as he ever has.