blowjob suffocation?!

Okay so me and my boyfriend LOVE to be ruff in the bedroom and when I give him head he loves pushing my head all the way down, since I have no gag reflex. I've tried telling him it gets very difficult to breath and that he needs to be more careful, but I think he does it without really realizing it. How do I control my breathing better to be able to do stuff like that in longer periods?