Giving head

My boyfriend likes it when I give him head but he is always asking me to deep throat, I want to but I gag to much, sometimes my eyes even tear up and I have to stop. I never last more than 5 mins. When I try to deepthroat I feel like I can't do it or I feel like Ima throw up. Any advice on how I could deep throat without feeling that way. How do I stop myself from gagging to much?