Seemingly itchy skin papules appearing nearly overnight?

Just when I thought we were done with the constipation issue, here's another thing coming up... I noticed on our 7 1/2 yo Siberian neutered male Kastor some bumps yesterday while petting him, and my partner showed up this evening to tell me the same - so we're pretty sure this is a recent issue. We've been away for the weekend and didn't notice anything before. They're small bumps (some very small) under the skin, some of them with very little to little scabbing (Kastor seems a little itchier than usual lately, especially around the neck, and he's scratching himself quite energetically, so it's probably coming from that). Of course, I've already googled around and checked this massive table of all cat skin issues and a few other sites, read on Bowen's disease, etc, and we're most likely going to advance his next vet time to this week or so, but I'd like to hear if anyone had the same experience of quickly appearing warts-papules-nodules-skin bumps?

Have you gone over him with a flea comb.  Even an indoor cat can get fleas, we bring them in.  They don't stay on us but they do stay on our kitties.  Some cats have an allergic reaction to fleas.

If he doesn't have fleas it still sounds like it might be an allergy of some sort.  Anything new in the house recently?

Thanks for your reply Denise. No new food except for the semi-new Fiber Response food which he got the most in the last couple of weeks (perhaps he built up the intolerance over the one month and a half he got the food?), also we were away last weekend and two of our neighbors, who also have indoor cats, came to take care of him. I think I got flea bites when taking care of one of them a year ago, so we'll need to look a bit more closely - although his undercoat is very white and I haven't seen any of the black debris that usually comes with fleas. Both neighbor cats are indoor, but the other one (another Siberian) goes out on a leash. Ours hasn't in a while - he doesn't like the snow all that much, it seems.

Other new things -- I can't think of anything.

Forgot to mention that the bumps are, as far as I can tell, all located around the base of the ears (perhaps 4) and around the neck area (perhaps another 4-5+) and I found one around the armpit area (although I couldn't check so closely, because armpit area...). They might have been there earlier, but unnoticed due to the thick fur, and due to scabbing became perceivable. 

New today: He lost some hair on the back of one ear, came off easily with small patches of dry skin (like on this picture). Some of the bumps have now a reddish dot in the center, but it might be the scratching.

We're going to the vet on Saturday, and he shouldn't get any dry food until then, so we'll see what happens.