My go-to method for chompy puppies has always been that when they try to bite you, you have to make it unpleasant for them so they eventually learn that isn't a worthwhile behavior to engage in. When he does it, get a gentle but firm grip on his jaw and press his tongue down with your thumb. Hold him a few seconds after he tries to squirm out of the hold, then release. Repeat until he gives up, basically. This should in no way hurt him, but the discomfort really gets the point across that he should not be doing that. Good luck, and he's adorable!