First, I'm amazed at how much information you managed to provide without using key vowels in certain words. Kudos. That's impressive.
Here's the problem, if he's awful, you not being honest isn't making him get any better. In fact, its inspiring him to keep making the same awful music. And what happens when he puts his music out there and other people tell him that he sucks or worse, don't pay him any attention whatsoever?
(There's a third option here where a terrible rapper ALSO manages to become an overnight success, so there's a good chance that as long as he's got a good enough gimmick, he might become the next Trinidad James...I can see your conundrum.)
You seem to be certain that he's going to look like an idiot so perhaps its in your best interest to start being more honest with him. It's in his best interest as well. Nobody wins if you keep lying to him.
Or the alternate scenario is that you help him as much as you can so that somebody else can tell him he sucks and then you can just lick his wounds as he one day decides to do something else since he's not good at being a rapper.
In fact, I'm fairly certain that's what most Aspiring Rapper Wives do.