Welcome to the forum! Theres alot of great, helpul people here!
I feed Samson Natural Balance's allergy formula, the Sweet Potato and Salmon one. Samson is an adult (4 yrs), 55 lbs, and for that size they say 2.5 - 3 1/3 cups a day. I always feed less than the guideline because bulldogs are so inactive. I feed him 2 cups but one of those cups gets fixed with wellness 95% salmon and salmon oil for his allergies so that adds calories as well. It's worked WONDERS for his allergies, no kidding! I would say to you to still feed him about that 3 cups. Maybe 4 is he seems really hungry or looses weight.
As for the chin acne, what kind of bowls do you give him food and water in? Plastic food and water bowls can cause that. Stainless steel is the best, or ceramic.