excessive licking and chewing?

About a week ago, I spotted a small wound on my 8 year old cats back. It had scabbed over, looked a little sore but he had made no complaints so I assumed all was okay. I put it down to that maybe my house bunny had caught him with her teeth because they play
fight a lot.
However, last night he was lounging aroundon the carpet when I heard him making a strange noise, it was really grizzly and he sounded distressed, I looked to see what he was up to and he was pulling at the skin on his back. I had to pull open his jaw to stop him from tearing his skin off. Luckily, he hadn't done any damage but I now realise I was wrong to blame my bunny!
Instantly, I thought fleas so i checked him over and his pure white so fleas are easy to spot on him. I found nothing. Any ideas on whats upsetting him?

Well on the face of it, it sounds like the wound was bothering him, either painful or itching.  It might be infected, but if he has re-opened it I would suggest a vet visit.  He will probably benefit from a topical ointment and may need antibiotics etc, but if he was opening the scab with his teeth, he's telling you he needs a vet.