Hi there

I think I last posted back in October of last year, my dad had Stage 2 skin cancer on his face. They cut it out and it seemed fine. Anyway he's had swollen lymph glands for a wee while, finally got the result yesterday that the cancer is in the lymph glands I think just in his face. He's getting a PET scan next Tuesday. Can anyone tell me what this is for? I don't know very much. They seem confident it hasn't spread to the other lymph glands but is this scan to check if it has? 
Is this Stage 3? What are his chances, can anyone tell me? He's 85. I'm trying not to panic.

many thanks