Sounds normal-when they are blowing their coats it can get quite thin before the new fur grows in. i would just be careful next time -not that brushing loose fur is bad at all-but careful of how much pressure you put on the brush. When the fur is thinner from shedding the brush more easily will touch the skin and could sting a bit. So maybe more sessions of brushing that are shorter would be better during a major shed

I think you can gently brush the thicker areas again. Just don’t go near the red area and keep an eye on it. Use softer strokes, that is, don’t comb down to touching skin but just comb out the hair itself. The reason it is helpful is that if you don’t, he is more likely to lick it off and ingest it, so you’re doing more good than harm. If you feel it pull, stop. And do a couple of quick brushings a day, rather than a prolonged one all at once. Then you will catch new batches that are ready to come off rather than pulling on those that haven’t quite been shed yet. If you are really concerned about the Hair Buster, just wet your hands and stroke him instead. The loose fur will come off in your hands.

Keep an eye on the redness. If it gets worse, you can call your vet. Tell her I suggested putting a little Bag Balm on the area as a salve, but I’m not sure a vet would agree rather than want to see him. Pink skin is not the same as a reddened area. Pink skin patches are OK.