I don't know anything about monitors, but when researching about beardies before buying one, I learned that incomplete sheds can cause them to lose toes. The old skin stays stuck around their toe or claw while the lizard keeps growing and it acts like a tourniquet. If that is the problem, a long soak may help get that old skin off, but you may have to work with him for awhile if it's been on there long enough for him to be losing claws. This, of course, is all a guess. A trip to the vet would be a very wise move.