Sounds like Grey horse Melanoma. Your vet can tell you.
It is one of the hazards of having grey horses (most, not all, grey horses develop them). There have been various treatments.. I recall one years ago that involved tegamet (Cimetidine). The Treatment, as I recall, was effective but very expensive and had to be used for the life of the horse.
I do not know if this is still a therapy used.
Have your vet check him and go from there. Meanwhile, keep your eye on the lumps to see if they change. If they can be seen from outside, take photos. Mark the margins of the lump with a magic marker and use something for scale (like a dime or a penny) next to the lump when you take the photo. It will help you monitor the lumps.