125 is bigger than 150? Sorry, I have no idea. S.O. doesn't even have cell service (I hear that's a prereq for deer camps?) but I don't want to miss this if it's an awesome deal.
There are all different kinds of cc's for bikes but the main for his size and age are 125 and 250 two stroke and 250 and 450 four strokes. The 125 and 250f (the f is for four stroke) are comparible in power where the 250 and 450f are comparable in power. I just saw where you said he had no experience so the 250 and 450f are out and like someone said earlier a two stroke is harder to ride so I would go with the 250f. You never did say what type of riding he would do, that makes a big difference. Track or trails or just to put around the field?