I know it's a little gross but maybe a lot of spit perhaps (if he is willing to do oral) or some lube. But, what also might work is not to stimulate it directly. Start with the area around it and work some of the skin around the clit rather than the clit itself. I have actually heard of a lot of women who can't take stimulation directly on the clit, it's just too sensitive.

For me, I'm an uncut guy, and for a long time it was really hard for me to get over the sensitivity of stimulating my penis by directly touching on it. Even now, I usually lube it up with something if I want to touch it, or I simply keep the foreskin over it. The clit is similar, it has a little hood and you may actually just be able to stimulate it comfortably through the hood instead of directly. It may sound weird, but a clit works like a tiny little penis, you can use the skin near it to "jerk it off" just like a man would.

You may also just direct him specifically on how you want it done. Try to mimic the way in which you masturbate yourself but allow him to do it. You'll only really need to instruct him the first time, thereafter he'll have the hang of it. Communication can never be underestimated during sex or foreplay. It took me a while to request or instruct too, because I didn't want the woman I was with to feel like she was doing something I didn't like. But, eventually, I made a system where I would say to her - "You know what I really like, is when you do it like that, that feels great" and then I demonstrate and allow her to do it.