Maybe try Van Patten's "Natural Balance" brand Limited Ingredients Diet.

My mini poodle was a tub-o-lard and having problems with ear infections. I'd been feeding her Pedigree for years. I'd heard that the ingredients and fillers in many popular dog foods can cause problems like obesity, ill health, allergies, and whatnot. So, as an experiment, I put her on this (she gets 1/3 c. twice daily) Van Patten's L.I.D., Venison and Sweet Potato. She's been getting it since last fall.

She's now a normal weight, her coat is beautiful, and her ear is much, much better. I'll warn you it's stupidly expensive- $40 schmackers for one 20 lb bag. But you have a little bitty dog, so it will go a long way.

If that doesn't work, maybe try feeding less. Or you could have him tested for thyroid issues.