Over-inflation of the lungs can cause the victim to vomit, blocking the airway. Turn him on his side, facing you, and sweep out the obstruction. Then continue rescue efforts.
Dentures can help create a seal around the victim's lips.
If your mouth won't fit over the victim's, simply put your hand over his mouth and blow into his nose. Blow just enough air to see the chest rise. Over-inflation of the lungs can force air into the subject's stomach, causing him to vomit. This creates an airway obstruction. If this occurs, turn the victim toward you, sweep out the vomit and continue your rescue efforts.