Since he is a teenager, I would hope that this is only the first of many aspirations he will have. I will be supportive and enduring. I may not agree with his choice of dreams, but I will be his biggest fan.
If I have reservations about whether or not he will meet success, you can bet I'll also be supporting his college education too!
I wouldn't try to talk him out of it unless I felt that he was making decisions that could be harmful to him. Unfortunately the music business can be cut throat and there are many aspects that could entice a young man to become part of a very seedy environment. This is where I would step in. Drug use, promiscuity, and violence are present in many parts of the music business and I would drag him out by his ear if I thought he was becoming involved in any of these.

Well my son is.. very very white. And though I am not racist.. white rappers do not have a strong history of success.lol Being that we also live in Alaska.. I'm gonna go with the idea would crush itself without my input. I would support my son in any decision but also provide him with the facts and reality of any situation whether they be for or against his idea. The fact is skin color aside few rappers make it "big". I would influence him to have a strong back up plan and let the idea play itself out.

I would try my best to be supportive of his rapping while still encouraging him to explore other interests. As a teenager, his aspirations are likely to change often so I wouldn't be too concerned about a seemingly unrealistic goal. I would not try to talk him out of it because my experience with teenagers tells me that this would only serve to make him want it MORE. I would just try to make sure he had a realistic outlook on his goals, maybe even have him see how many struggling rap artists there are, while still letting him make his own decisions. I would also insist that education remain a priority, regardless of the rapping.

Sure. You can support him. I wanted to be an astronaut when I was in high school. You can see how far I got with that. It may be a passing fad or it may be something he is really interested in. It's hard to tell with teenagers. It could be because he has a friend (or he likes a girl) that is into rap and now he has taken a keen interest in it. Except for having to actually listen to it in the house, it's probably not any worse than any other career aspiration. If he enjoys it enough to maybe follow a career in music in college, then he will be exposed to a variety of different musical styles. He may not limit himself to rap but might just start there.

Maybe you can try to direct him to expand his vocabulary too?

from http://www.songsforteaching.com/fillyourhead/iaintgot.htm:

I Ain't Got (A Good Enough Vocabulary)

--quote--
She's so loquacious,
tenacious,
sagacious
- but not too spacious.

She's inscrutable, beautiful
- it's irrefutable.

Never meek with us, obsequious,
salutary when she speaks with us.
Munificent,
beneficent, yeah she's magnificent

But she won't go out with me – because
I ain't got a good enough vocabulary
No-no-no she ain't got time for me – because
I ain't got a good enough vocabulary

Word -- no rhetoric -- word --
She'll often elevate,
elucidate - wish I could get a date -

Felicitous,
solicitous -
her smile's ubiquitous -

Persistent,
consistent, only somewhat distant -
un-suspicious, does the dishes,
bet her kisses are delicious.

She's benevolent,
relevant,
even grandiloquent -
perceptive,
reflective,
and circumspective

She's imperious, I'm serious
- makes me delirious

But she won’t go out with you because
I ain’t got a good enough vocabulary

So I been workin' on my definitions,
long divisions too,
In case she also needs someone who's good in
math.
And by reachin' way up -
for a higher education I’ve been changin'
the trajectory of my path.

Now I'm tenacious,
audacious,
so loquacious -
I'm magnanimous - plan on this
- never pusillanimous

But now I won't go out with her - because
She ain’t got a good enough vocabulary

Now that I understand her she's pernicious,
malicious and
surreptitious -
nefarious, the scariest, she's Aquarius,
inconsistent,
vitamin deficient

She's dogmatic,
erratic,
idiosyncratic,
she’s petty and sweaty - like who?
- like your cousin Betty and

You won't go out with her because
she's got an arbitrary,
elementary, rudimentary,
fundamentally limited voca - bu - lary!

What is up with this word spendthrift?
--unquote--