Honesty is the best policy.

"Listen, (his name), I am uncomfortable about this whole dildo request, but I am open-minded and willing to try it once. If I don't like it after I try it, I'll let you know."

Then it is up to you to figure the following: yes, it's weird, yes it's kinky, but nobody will get hurt, so why not try it?

You may be uncomfortable doing it, but at least you gave it a fair shot, and heck, you may surprise yourself and get into it. The only way to know is to find out.