I think using reverse psychology is a mistake. Might work in sales but i doubt it'll work with kids.
If he actually likes tennis, try to find out what he likes by observing or simply by asking him. Be motivating and positive and focus on what he does well. Also make sure there's enough variation during your lessons. Kids have a smaller attention span than adults. The focus should be on having fun rather than improving their game (though try to incorporate drills for technique development), unless the student is very eager to become a better player.
But set your boundaries and guard them, because he might test you (thats what kids do). If he crosses them regularly be clear that it's not ok and maybe have a talk with his parents.
Working with kids is a lot of fun, but it can be very tough.
This kid has gotten better and I didn't use double reverse psychology. Instead, I have allowed him to be a bit eccentric which he does for getting attention and a way of exerting power. Since I am doing a semi private lesson I just ignore him and praise the other kid who is really trying. It appears to me that he does things wrong to get me to correct me or to make a mockery of what he is doing. I ignore him and say things like, "keep working at it and eventually you will be able to do it." I then refocus on the other child. I know the other kid can do the drill, but chooses instead to seek attention. I don't give it too him and my little remarks he is not sure but what I am sincere in my comments. He doesn't like it when it seems like I am not noticing he is goofing off on purpose. This seems to be working.