Wouldn't you have to pay for the more expensive board at the lesson barn for the months you were planning on sending Casper there for training anyway? Plus the training fees on top of that?
If you are confident your son will stick with riding and is most interested in doing hunter/jumper, this one that's for sale sounds like a good match. It might be worthwhile to do a trial ride ride on it (during a lesson would be ideal!) and you could see if the owners are willing to do a lease with option to buy. And just because your current barn doesn't have openings right now doesn't mean they won't have one in a month or two- ask the BO there if they have a wait list or if they know of anyone planning to move out in the near future.
A few things...
1. My son is already taking jumping lessons on a very dependable and safe lesson horse. He is not ready to jump on his own yet (I mean outside of a lesson) but maybe in a year or so he will be.
2. The barn we have our horses is not a boarding barn. It's a woman who has 20-30 horses and is just letting us lease two of them. Her sister is my best friend, so I don't think she'd be leasing to us w/out that connection. She does want to sell some horses and may eventually have room, but that won't be for a bit. Some are being trained right now and hopefully sold once their training is done.
3. I don't think, if my son seriously pursues jumping, Casper will be THE horse. I'm thinking just for the beginning years. If he turns into a kid passionate about it in his teens, we could look for a hunter/jumper at that point, and probably a new lesson barn that focuses on that. Our current trainer rode hunter/jumpers when she was young, so she teaches basic jumping, but her main focus is the AQHA shows.
4. And yes, I'd have to pay $$ to have Casper trained for a few months and expensive board and lessons. I can afford that for 6 months or so...it just isn't something I want to commit to long-term.