I don't understand.
Why..? Compression? Reducing dynamic range makes a problem like this worse.
And you're buying in to the 'dynamic mic' being a solution?
Like I said before, if he has the Mic in front of his mouth and the guitar still drowns it out, either he is singing too quietly, or he is singing into the wrong part of the mic.
I'd just counter that quite often in these scenarios the dynamic mics, partly as they're designed and partly as they are used, the vocalist may work a mic closer than they would with a condenser.... Well I watched a video and the dynamic made sense due to not recording the room and some separation from a better off axis rejection. of course thats one approach ..