What's going on here?

[SasherinSuite]

Doesn't seem to be causing him any discomfort but he is grooming alot and the fur is patchy like this all over his back.

Looks like flea dermatitis. Or maybe lice. What flea treatments are you using?

Get a wet piece of A4, hold him over it and rub his fur so any bits fall on to it. Flea diets are dried blood and will form red streaks on the wet paper. If you see anything crawling, catch it on some sellotape for identification by your vet.

We use Frontline on him. He doesn't appear to have any fleas though.

In the absence of anything crawling, probably best to vet him.

Could be a skin allergy. Might be a reaction to flea treatment.

Hard to know without tests.

Could also be stress related due to changes at home, or if he is elderly he may have arthritis in that area.