Don't expect the action of your mouth to be sufficient. To avoid teeth scraping the sensitive glans, lay your tongue over your lower teeth and make an O with your mouth. Practice by yourself a few time with a banana or cucumber. At first it feels awkward and your mouth will tire, that's when your hand fills in while you relax mouth and tongue muscles. Along with working the shaft of his penis with one hand, occasionally take him inside your mouth as far as you can.
Deep throat is everyman's fantasy.
In order to do this, you must learn to make friends with the gag reflex and simply go with it. You will not throw up or choke to death. Again this is a matter of learning to relax your throat muscles and to see the gag reflex as a wave you can ride, not a life threatening event. The fact that eyes tear while drool runs down your chin only makes it all the more hot and sexy. For variety, applying pressure to his perineum, the space between the testicles and anus, feels good.