The rapper should spit his style and flow to this particular beat. Might be a bad idea to tell him to spit in a certain way, he should spit in the way he's most comfortable. At the most you could perhaps hint that he should spit with more emotion if that's the problem. And make sure he knows his lyrics so he don't have to read it from a paper, cause that often sounds like it as well then.
If you feel it's missing something, might be a delivery problem, but most of the **** you hear on releases are pretty heavy on adlibs and double takes. So that might be why you think something's missing.
A verse is often recorded in one continuos take, and then double takes and adlibs added. But sometimes it's more cut up, there's a lot of tracks out there where the verses are recorded bar by bar.
Sometimes even whole verses are double taked and layered, and more often than not there's more words in a verse that's double taked than not. Especially in mainstream rap, underground rap feature slightly more raw verses though, single takes, no doubling, etc.
Check out some of your favorite tracks and see how it's done there. Listen intently, be focused and really analyze the tracks.
Some rappers are just great, you don't need to do much to their take. Sometimes less is more, others might need some help to make it interesting.