I'm a teacher, and I'd suggest that you reconsider this plan. Why should you (or he) be out-of-pocket for a degree when the school system is willing to pick up the tab? In my area, this choice is available to any teacher who has completed three years of teaching. The teacher pays for the classes up front, and is reimbursed after passing the class (fail the class, no reimbursement).
Beyond the Master's degree (which adds 10% to a paycheck here), there are a couple other ways to boost salary:
Earn National Board certification. This adds another 10-12% to the paycheck. This is a difficult certification, and the process cannot be started until the teacher has 3 (or is it 5?) years in the classroom. The certification lasts 10 years and can be renewed. Earning NBC is a one-year process and is very expensive (about 2K, I believe), but the state will pay for the teacher to go through the process once. Many people do not pass, so I would suggest that a potential candidate teach a couple years and earn the master's degree (so as to be up on the latest jargon, etc.) immediately before embarking on this expensive process.
Become an administrator. Here's the hitch in getting that master's degree straight out of college: If you want to become an administrator, you want one degree; if you intend to stay in the classroom, you want a different degree. If I -- a classroom teacher -- had a masters in administration or school counseling or library science, I wouldn't get jack-squat for it. I would only be paid the extra 10% if my masters were in my subject area. So the point is that you want him to earn the right degree for what he wants to do in his teaching career. Until you have a couple years of classroom experience, it's hard to know which one you're going to want to earn. And then there's a program called "Principal Fellows", which pays highly qualified would-be administrators to earn a Masters degree AND it pays them a salary while doing so. I used to think I'd do this eventually, but now I know that I love my classroom too much -- I'm where I belong. LOTS of teachers would tell the same story; some go ahead and plunge into administration, knowing they're not going to like the job, because our pensions are based upon our last four years in the school system.
Seriously, while I understand your thinking -- he's going to be working, why not earn more for the same job -- I would suggest that a teacher get some experience on the job first, then when he knows what he wants, let the state pay for it.