[@Proleptic]

Mutant gene activated by the radiation is what I was going for. Now for his abilities and what not I could make it so that he has amnesia or that he had just been laying there unconscious all this time, considering NO ONE is actually allowed to visit the site it would make sense and considering it was a reactor meltdown and he was beside it, the initial wave activated it, the second killed him but his regenerative ability brought him back but it took a LONG time? 

The first paragraph of the mutation could stay but the rest is dormant (trainable?)