First things first, if you suspect that it is ringworm, DON'T share brushes, or any tack with any other animal, and wash your hands after touching him, it can also be spread by direct contact between horses, so if you can keep him from touching other horses then do it.
Ringworm isn't a worm as such, it's a fungal infection/dermatitis and is similar to rainrot, and it will often show up if the horse has inadequate immune system or is low in vits and mins. Horses under 3 tend to get it more so than Adult horses because the immunity of that horse hasn't been built up to it's maximum yet. So you need to be treating it with an anti-fungal cream or wash of some sort and just keep an eye on it. Using an anti-fungal shampoo all over your colt would probably be your best bet.
It can be spread not just to other horses, but to dogs, cats, pigs, sheep etc, plus yourself.
Can we have pictures? As my mare had something that was described the same way you would describe ringworm, but when shown pictures it was obvious that it wasn't ringworm.
toothpaste works because it dries out the ringworm, ringworm favours warm wet conditions, and toothpaste dries it out the same way it dries out pimples on humans.
It also hates airflow, so the more air it gets exposed to, the better the chance of getting rid of it.
****, my vet nursing courses are making sense already