Thanks. I'm not sure if it's the same because language classes are a minimum of 12 hours a week, but Chakrit website says minimum 2 days a week (not sure how many hours per lesson/day).
You don't need to convince anyone unless and until they start asking questions, which won't be anytime soon. Just have proof of funds (cash!), residence/hotel, and onward flight ticket. Beyond that just tell the truth and he'll be fine.
Not sure why some people here are so hellbent on SETV. Either way will work, and either way will be about the same amount of hassle. If anything my personal experience with extending was no hassle at all- in and out in under two hours typically.