Herge,

I'm not certain that there is a volume pedal that can accomplish this but then again, there are many new toys arriving every day in music stores. I don't believe volume pedals for instruments will work the same way for a vocal mic. As others here have stated already, there are means by which one can use such items to solve the issue but the cost may become prohibitive.

Here is my simple, no cost solution to your issue. This could and should work but it may depend upon your PA system, band configuration, type of music you're presenting and perhaps even the kinds of venues you are playing. You've not mentioned any of that here, only, singer, banjo, band, and SM58. It's hard to offer a better solution without more facts but you can't get any cheaper than a free solution.

Here's mine. Could it be possible to merely turn the volume of the mic up to what is needed for the banjo and then when your singer sings he can simply back off the mic to reduce volume level for his vocals? There you go, a free solution, that doesn't hurt your wallet. Let me know if it works, or if you've tried this idea already.