Maybe you need to let him fail.

This may sound harsh, but we as humans often need to learn a lesson the hard way. Now is a good time to fail because his grades won't count for college. And even if they do, sometimes a person still needs to fail.

IMO, there isn't much that an adult can do to convince a child that s/he needs to work or work hard. This motivation has to come from within, and in some people, it can only come after experiencing what the alternative is like.