You may not be hard enough on him. Grounding normally means the child can be in one of two places, school or home. There is no visiting with friends unless it's under your roof and with your supervision. When I was growing up, I knew when the curfew was and I didn't dare be late without a very good reason.
Never make rules unless you have a good reason and don't allow them to be broken. At 13 years of age I would think a 10PM curfew would be more than fair as your child probably should be getting up around 6-7 am even during the summer.