If he's truly determined to become a pro-gamer, I'd bargain with him. If he completes his STAT and scores favorably, he can have a gap year to try - PROVIDED he also picks up some casual part-time employment (even 5 hours a week would do).
Many many people go to college/University before they have the maturity and life experience to decide where they want to end up. I strongly believe that students shouldn't be forced into tertiary education until they have a better appreciation for working life, and how difficult it is to make one's way in the world today. At 18, so many still have an exceptionally inflated ego that produces totally unrealistic dreams. This gaming ambition of his is almost assuredly a symptom of that. Funneling these sorts straight into tertiary education, wherein they will accumulate debt and very likely chop and change courses, is just about the worst choice one can make.
Source: speaking from experience. I started in Arts at 18, aspiring to become a famous author. Hated that. Transferred to a Bachelor of Agricultural Science - a degree and field I never would have imagined myself in at 18 - at 19 which I have recently completed at 22. Was accepted into Veterinary Medicine for this year, and have since decided I'm going to go back to University for another year to hopefully bridge into Medicine or Dentistry. Its taken me 4 years of post-secondary education, and casual employment to work out what I really want in life.