Blow job advice/tips

This guy wants me to give him a blow job, I want to satisfy him but he likes deepthroating and swallowing, I have a nasty gag reflex so deepthroating is out of the question. I don't want to swallow because I'm worried about STDs, so he wants to cum on my face. I guess 2 questions here: how can I give him an incredible blow job, and how do I avoid risk of STDs while still giving him enough of what he wants?