Mystery bumps on a bald patch!

[Iznurgle]

I have a 5yo 14.3hh Connemara who I'm bringing on and fittening up, who has been an absolute dote to work with. He's a rescue, and was rugged quite early last year, and was also put in a 6 foot rug when he's more of a 5 foot 6 kinda guy. He's been with me for about 3 months, and when he arrived, he had quite bad shoulder rubs. We're taking things nice and slow and at his pace, working in an arena doing flatwork schooling and small jumps, hacking out, some lunging and long lining, and in hand work.
As part of our fittening regime, we've been working on spurts of canter work, and on Monday after working he was sweating a little for the first time. The skin on his right shoulder (not pictured) had what looked like a heat reaction, but the left came up in these bumps - still here today, though smaller. I turned him out without a rug for the day, as it was dry and moderately warm, and gave it a wash before leaving it be. It looks like a reaction to something, but I've never seen anything like it before. Honestly, I'm a little stumped.
I have the vet coming out tomorrow to do his vaccines, so I'll ask her about her thoughts, but I was wondering if anyone has any idea what it could be, what I can do about it, and also how best I can encourage the hair to regrow!
The bumps are hard and dry feeling, but he has no pain reaction to me poking at them. He's fed ~50g pink mash unsoaked, scoop of lucerne mix chaff, and a handful of cool and easy cubes, supplemented with biotin and karrion oil, served soupy. He's on ad-lib haylage when in, and has been for the last three months at least. Out 8am-3pm, was previously out 24/7, light work ~5 days a week.

 

[ycbm]

I've seen similar. It was a sweat rash and the dead skin peeled off with all the little bumps. My mare gets a tiny bit of what looks like foliiculitis under her saddle, too.

I think it's a bit out of favour recently but I wash with a mild solution of hibiscrub before and after working and it stops it.

 

[Iznurgle]

The hibiscrub suggestion is excellent, thank you!
The vet came out today and thinks it may be fungal, so we've been given a pink antifungal powder to wash with (the name of which his secretary was unable to give!). He hates baths, so even pour warm sponging was a struggle, but he's to have one every day for three days in a row, so he'll either have to get used to it, or baths will be ruined forever!