Quite a lot of dogs get acne. Most commonly it is an adolescent problem (no surprise), but it can persist into adulthood and boxers seem to be a breed more prone to that than many.
Things that help are using stainless steel or ceramic food and water bowls (never plastic) and washing them frequently in the dishwasher. Likewise, wiping and drying his face after eating is helpful. If needed, you can apply a weak hydrogen peroxide solution to the spots. Here's a short article on acne in dogs that might be useful to you: http://www.peteducation.com/article....&articleid=878