For most adolescents, the problem is that body odor is embarrassing, and socially unacceptable, and hard to get rid of. But for teenagers with special needs -- such as sensory integration problems that cause them to be insensitive to bad smells, for example, or developmental delays that give them a young child's lack of concern over young adult issues -- the problem may be that they really aren't bothered by BO at all.
If your teen's got an odor that's unavoidable, and a lack of motivation for getting rid of it, you may have to take the problem under advisement yourself. First, check with the pediatrician to make sure there's no medical reason for excessive bodily aromas.
Then, see what you can do to make what's socially acceptable more attractive to your little stinker.