If you don't feel comfortable masturbating in front of him, then simply tell him what to do. Having better communication during sex can improve your overall sex life.

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Here's an example:

You guys are getting hot and heavy with some kissing, and his hand starts to creep down to your crotch. You immediately think the worst, because you know it's not going to be a good time with his rough hands that seem to have no control.

Instead of dreading what he is about to do, lay your hand on his and move it the way you like it and want it. Let go at times to see if he gets the hint, if he doesn't, lay your hands on his again to show him what to do again. You can even whisper, "I like it like this...more like this."