Sounds likely to be just retained dead sole to me. If his feet are perpetually locked into shoes and the farrier doesn't remove the excess, then it does tend to build up.
Perhaps it's been particularly dry there & soaking his feet will soften the dead material to allow it to come out more easily. For the health of your horse's feet, don't put topical goop on them, unless you occasionally want to pretty them up for a show or such. Diet & nutrition are important factors of healthy hooves, so yes, an appropriate supplement to balance your horse's diet is a good move.