I haven't read the whole thread, so I'm sure these comments have been made by 20 people already, but...
If you have a designated band leader, they need to talk to the drummer. If you're more of an autonomous collective, anyone (or everyone) can do it. The conversation needs to be calm and constructive and explain the importance of tempo to the song, and that he needs to be able to set it properly. The real purpose of this to determine whether the problem is (1) that your drummer is incapable of setting and maintaining a tempo, or (2) he doesn't give a $#!&. If the answer to either of these questions is "yes" you should probably fire the drummer.
If the drummer can carry the beat, and wants to, the next step is a click track. There are plenty of free metronome apps for iOS and Android, so get one downloaded, set the tempo you want for each track, and feed that into one of the drummer's ears. If he still can't get it after that, you should probably fire the drummer.