Dun will dilute the coat but along with dun come two other factorsthat have been identified at that location nd1 and nd2. nd would be non dun. The difference between 1 and 2 is that 1 will put primitive markings, possible slight dilution and can cause excessive fading. 2 is no primitives and no dilution.
He looks like nd1 as the dilution is not strong enough for Dun and I do not see a dark enough dorsal that goes through the tail. The white looks like scarring from scratching, rubbing or a fungal issue like rain rot.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.