If you really want to illustrate your point about theory try asking your drummer to accompany some Cynic or something. There's a lot of odd time sigs in there but it is still really full on metal. The drummer's name is Sean Reinert, he is the daddy. He also played on the Death album, Human ( I think the dude was only about 18 years old when it was recorded). I've been playing for about 11 years and am really into odd shit and this guy makes me want to puke into my stick bag and take up the maraccas
To say that time sigs aren't important to metal bands is a bit like saying that sweep picking has no place outside of prog bands. The only people that really think that are the ones who can't be bothered to learn. The theory behind time signitures really isn't hard to figure out, I think your drummer would be able to master the idea pretty quickly if you could only persuade him that it is worth the effort. knowing theory really won't hamper his creativity, in fact the more theory you know, the more creative you can be because you have the freedom to express yourself in so many more ways. (How many different ways can you play 4/4 anyway?)