I plan on letting my boyfriend know that he can not be a rapper.
What is your reason for feeling like your boyfriend is not able to be a rapper?
He's really not talented enough to be a rapper.
I'm sure you don't feel like your boyfriend is talented enough to be a rapper, but you should be supportive of his dreams. As his girlfriend you should find ways to support and encourage him.
He has no chance of being successful as a rapper so why should I bother.
It is a bad idea to tell your partner that they can not do something. It would be a better idea to find ways to encourage him to chase his dream.