I think your best bet is to get an appointment with a dietician who will work with Derrick to develop a diet based on his likes and dislikes and may have ideas about how to work caffeine into it.

Those "no added sugar" fudge pops are awesome! And the serving size for the small ones is actually 2. When I was dieting, I found them very satisfying.

A&W diet rootbeer and cream soda are also really good. They taste exactly like the real thing, but I don't know if they have caffeine or not tho.