I've been using hoof armor this summer and am only using boots in front for long rides. 75% happy with it. If he didn't have wonky action on one leg that wears one side down when going faster than walk on roads I'd get away without boots.
Have you been riding and he's reacted to what you washed your saddle cloth in? Just a thought.
I wouldn't put anything on it, it looks like an allergic reaction and you don't want another reaction to what you put on it.
Definitely get the vet to test asap for Cushing's and EMS. If you find a metabolic cause you can treat it. It may be a good idea if you can find another yard that is set up for laminitis horses where they can be together in a bare area eating soaked hay.