I take a kick-boxing class at school, so I got a few tips for you.
Keep your fists clenched, and right below your eye so you can only see straight, or up. Let him take the first punch, and when he swings just tap it out of the way with your forearm, and the jab him in the face. When you block his punch, just tap it, don't fully extend your arm. Just a little nudge will do it.
If you get the chance, grab his head while still protecting your face with your arms, and knee him in the nose.
Just keep in mind, always protect your face.
Hope I helped.