I'm going to recommend you put the horse shopping on hold, either lease or purchase for at least a year. Better to gain experience riding different horses. I've worked with people who rode as a kid and they were the hardest to work with. They ride how they think they used to ride. When I'd try to get them to ride correctly they'd promptly revert. Your son may seem interested now but that could change. He's at the age to trying new things. If he's pushed he may more on to something else. Delicate balancing act.
You calling me hard to work with?
I know what you mean though. I learned at such a young age...probably 4 or 5, so it just naturally happened and I was never really taught. I'm sure I have a boatload of bad habits. I am switching over to English/hunt seat from Western so that transition has meant most of my old ways have been thrown out the window.
You are right in that my son may change his mind. He's 11, so who knows what those teenage years may bring. I am definitely going to wait until spring to make a decision. He will have been riding for a year at that point and I'll have a better idea. Leasing seems pretty safe to me though, even if he loses interest.