What could have caused this?

[Toddlerteaplease]

(My parents boy)
They noticed this lesion on his back a couple of weeks ago. Had steroids from the vet. It was getting better, and the fur growing back. But had now flared up again. He's got antibiotics and some hibiscrub to clean it.

We can't work out what has caused it. We did wonder if he been rolling in the rock salt used for gritting, it has coincided with the bad weather.
Any ideas?

My cat is prone to these "hotspot" skin infections. Always in the winter, they seem to flare up when she cooks herself near the radiator.

They grow very quickly; a tiny patch of sore kin becomes a red, weeping and angry sore that seems to grow by the day.

Has he been tested for ring worm?

I'm not sure but he's up to date with his regular wormer and flea treatment. He's being threatened with a body suit if he starts licking it. He's not impressed with that idea. (And I don't envy the vet trying to put it on him)

Bump