Breath Control

Recently a top I've been seeing has been using breath control on me. At first this involved only pinching my nose shut while thrusting his cock down my throat or putting a rubber hood without a mouth opening over my head and then closing off the nose holes. Last night he put a plastic bag over my head while I was tied up and held it closed until I passed out. When I expressed alarm about this activity, he assured me that he was in control and even said that next time he would introduce me to garrotting. I'm nervous about this. The guy is a great top, and I want to go on seeing him, but how do I get him to stop this?