So, is the diagnosis for allergies? 
 
Is he scratching his belly hair off, or just losing the hair without scratching? Any other issues at all...medically, that you notice? 
I'd d/c the oatmeal shampoo asap, bc oatmeal can actually irritate the skin. This would be better, imo: 
 Amazon.com: "DermaPet" MalAcetic Shampoo for Dogs...
Amazon.com: "DermaPet" MalAcetic Shampoo for Dogs...