White Flakes When Brushing

I know its not a great picture. But recently brushed my cat with a flea comb after spotting a large bug of some kind. (Not sure if flea, lice or what) there was only the one and other than 2 bits of small black bits, hes clean, save for these flakes.

Now I'm not sure if they're flea eggs or nits or dandruff at this point. He has been flea'd recently and the vet said he shouldnt be done again.

Hes not scratching anymore than normal, I did give him some 7+ wet food along side his dry food as the vet said it may help his weight. Other than the flakes, hes acting normal.

My concern is that we brought an injured stray in the house Friday night, it was put in the bathroom and left after 15mins (it was taken to a friends so RSPCA could collect).
Does anyone know what these flakes are and of its dandruff, what's the best way to treat it?

It is hard to tell from the picture, but the small black bits that you described could be flea dirt (flea poop).

If it is dandruff (white flakes), weight loss should help. And overweight cat cannot groom as well, so they tend to look like they shed more and will have dandruff as well. Does his fur sometimes seem a little coarse, or oily, especially on his back?
When it comes to the black bits, it could indicate flea dirt, but IDK.
I would go over his entire body, splitting his fur right down to the skin to do a full check. If you see more black bits, maybe a flea bath? and clean the room you put the stray in from top to bottom? I'm petrified of getting fleas in my home so it might sound overboard, but I've never had to deal with fleas either, so I'm not the best one to give advice on fleas.

I agree with Moka that the black pieces that you describe could be flea dirt. Use a wet towel and gently dab the flakes, get them to stick to the towel. If they turn red, then it is flea dirt (as flea dirt is digested blood). If so, you’ll need to get some flea preventative like Advantage or Frontline.

Thank you for the responses.
I have spilt his fur and I cant see any more black bits. He actually grooms quite a bit, after meals and naps. Hes a white cat and has only had fleas once before, and it was easy to spot through his fur. The white flakes on the other hand are harder to see. I only found then because of inspecting the combed fur.
We've sprayed the entire flat with flea stuff to be safe and I did the bathroom the moment it left mine.

I've been working of weight loss for a while hes only slightly over the weight he should be and hes indoor, so hes exercise comes from playing. Which hes doing a lot of.
We use advantage which we get from the vets, he was done 2 weeks ago.
The white flakes seem different shapes, not sure that helps.

Do the white flakes seem to come specifically from his back / along spine?

No they're coming from his sides too

OK, the reason I ask is sometimes, even if just a little overweight, the back (and sometimes the sides of belly) are hard to get to for grooming, he may groom well, but just not enough in those particular areas, because he can't quite reach or reaching those areas are a bit more difficult so he just gives them a few licks.
I had an overweight cat with dandruff, and he would have to rock himself and get momentum to reach his sides, and he never reached his back very well. Losing weight was difficult because of a thyroid condition. I purchased cat wipes to keep help him keep clean, and it did help with the dandruff too.

I found that brushing our cat who has heavy dandruff at least twice a day eventually takes care of the problem.

My long furred cat had quite bad dandruff a few months ago. I bought salmon oil and added one spray to her food twice a day. There was a noticeable difference in a few days.

The white flakes could be dry skin, but you would really need a vet to look at them under a microscope to be sure. There are other things that can cause skin to flake.

I have found in the past that dry skin can be seasonal. I don't know where you live, but I run my heater all winter and everyone (both humans and cats) sheds a bit more dry skin in that season. I have also found in the past that switching from a dry to a wet food diet helped reduce problems with dry skin that a previous cat of mine had. The wet food you are feeding now should help a bit, if dry skin is the problem.