Hmmm....well I have 2 teenage boys and they're different DS1 is now 17 but has always been pretty good at being clean even at 12, don't get me wrong if I didn't remind him to get in the shower he would quite happily not bother but that was more a forgetful or just not thinking for himself thing.
DS2 on the other hand, now 15, sounds like your son, at 12 would happily go without a shower for days if I let him, doing the old get in and just stand there after being asked 100 times to have a shower, wetting the hair and saying he washed it etc...OH and don't even set me started on teeth brushing!!
You'll be happy to know that at around 14, when girls came on the scene

the interest in hygiene picked up, although if there was no girlfriend, there was no shower
lol
Now he showers everyday (girl or no girl

)...But hair washing is still a struggle I think the "look" he's going for is easier with dirty oily hair I have to make him wash it before a haircut, my friend is a hairdresser and I don't want her to have to touch it in that state
So I guess it varies, they're all different, I'm sure it will happen for you, one day. good luck