Is he interested in history at all? Languages in general? Anything else you could see as related at all?
I studied Latin all the way through to A-Levels - mostly because my middle class parents considered it part of a "proper" education.
While I initially hated Latin with a passion, it actually became one of my favourite subjects in the end. What got me interested was that I was beginning to understand how much I could learn about how language worked and how I could find the same patterns and principles over and over again in English, German, French, Spanish, ...
Around the time when I was supposed to be confirmed I also developed a deep interest in the history of religion in Europe and loved being able to read the original texts of religious philosophers etc.
Finally, we actually read Ars Amandi in class. I guess there's no more perfect bait to get teens interested - although it really is a very strictly PG16 kind of text. A friend was into cooking and really got into finding and following recipes from ancient Rome.
What worked for me was really just discovering ways in which I could actually relate Latin to what I thought was worth spending my time on. IME (and I speak six languages, so my experience is pretty extensive) trying to learn a language is bound to be painful and often futile unless the motivation is there, so that's what I'd try and work on.
Also: If your son is into comic books at all: I used to have an entire collection of Asterix books in Latin and used to really feel super geeky smug about my entertainment.
Not sure if any of this helps, but maybe some of what worked on me may work on your son.