Punish Him with Corner time. Make it an effective part of your discipline.
You can assign corner time before and after another punishment (like a spanking or other) and it should always be with his pants lowered to below his knee clearly exposing his panties.
Corner time should be for about 10-30 minutes, and he should not be allowed to touch himself.
After a few minutes being sent there he should start to reflect on what got him there and stupid he was to get punished, and if it’s before a spanking, he should be getting really nervous.
All the while you are off watching television, chatting with friends on the phone, or just doing normal things.
You can do this alone at home, or with even more impact in front of a visiting girlfriend!