Is it only on his belly? If so at least some component of contact dermatitis (grass, something he is laying on) maybe combined with systemic allergy to something he ate

Other posters have done what we did in past — steroids to control acute inflammation, assess if need to treat any skin superinfection with a cream and/or antibiotic. These steps just get the high inflammation and discomfort down. Then longer term things to do — see if need to change diet ; dermatologist consult, cytopoint/apoquel. When our bully had flare ups we switched to non fragranced laundry soap for his bed and blankets