Scooting may ad to anal gland issues, my thoughts. If he's had rear end messes maybe it's just his thing., a habit
that can break in time make sure his diet is solid waste
Not an expert but from reading it may help to offer home cooked meats, shredded chicken {no bones} they can get constipated on dry. Fish with rice, few times a week. Pumpkin is great as you say. Holistic dry food helps with tummy issues or allergies. Sterilizing boxes thoroughly once a week...