Cats face a lot of the same skin problems that we humans have to deal with, including rashes, acne, insect bites, allergies, and other types of irritants. It can be difficult to know where to even begin diagnosing a skin problem.
The first step in diagnosing skin disorders is performing tests. Our first recommendation is to take Cokey to see a veterinarian who can help to diagnose the problem. That said, there are a couple of red flags in the things you mentioned.
Fish is not good for cats, despite the stereotype (and the fact that they love the taste). Many cats develop allergies to fish, so we recommend staying away from any foods that include it. The same goes for milk. Most cats are lactose intolerant. It's strange that these two things have been closely associated with cats for many years, but they're actually bad for them. If Cokey enjoys milk treats, there are milk products on the market like Cat Sip that are specifically made for cats.
You also want to look for parasites. Bumps like you describe can be caused by a flea allergy. If Cokey goes outside, it's a safe bet he's been bitten. If the bumps are itchy and you see him scratching a lot, look into better flea control. We recommend oral flea prevention products. Lufenuron is at the top of our list because it has no known side effects and is available at a low price from littlecitydogs.com. Comfortis is also good, but it can be expensive. You can see more about our flea control recommendations here: http://kittyhelpdesk.com/help-desk/flea-control-for-cats . As always, we recommend talking to your vet about any drugs you intend to give Cokey.
Cats don't usually experience heat rash, as most are descended from desert-dwellers. Of course, since you're only seeing bumps on his back, Cokey could have come into contact with an irritant when he walked under something. If that's the case, washing him down with a warm washcloth could help remove any residue that remains on his back. It's just a shot in the dark, but a gentle washing certainly wouldn't hurt.
I hope this helps, Pete. All our best to you and Cokey!
We received the following response from Pete a short while after we sent him our suggestions.