In the midst of your moans, say keep going, or right there. That's what I do to keep my man focused. He probably doesn't know what's happening.
Also, to kick it up a notch, get him to finger you at the same time. He can stimulate your g-spot and your clit at the same time. It will probably make you orgasm even faster.
One of the best things I ever did was explain to my man exactly what to do to my clit. I showed him where it was and showed him how to pull the hood back. Now I come so fast when he eats me.