I feel for you, bro. I had the same issue with my old guitar player. I succeeded in getting him to at least put the amp on a stand and point it at his head and away from me, but I was unable to get him and the rhythm guitarist to rearrange the room. Ultimately, I set up a practice space in my basement that was better suited to achieving lower volume. I have the ability to put the instruments in the board/monitors and monitor through headphones if need be. I also invested in a good pair of Westone earplugs.
If was in your shoes, I'd tell the other guys -- "Hey, I'm having a real issue with the volume wars, and it's getting to be no fun at all to come to practice. I'm asking that you trust me and let me *TRY* rearranging the gear so we can all hear better. I really think it'll make a big difference and allow us to get tighter as a band. If it doesn't work, then I'll be open to other suggestions -- but honestly, the current situation is not working for me."
As others have said, I'd rearrange the gear by placing everybody's cabs across the room from them. If the guitarist can't hear himself, then he may TURN DOWN your amp until he does. Something tells me he's the type who insists he must FEEL the noise to play his best, In which case, you're screwed.
Also, adding some acoustic treatments to your space may help: carpets on the floor, hang some heavy-duty moving blankets, or buy a bale of Roxul insulation at Lowes and make some panels. Anything to deaden the room and reduce the echo.
