17yo is half the problem here. $1500 is a good price for a good durable hard tail bike right now, and that's the other half of the problem. If he's done growing upward, size is sorted, but you really have no idea if this is something that is going to stick or if he is going to leave it in your garage the next five years while he's off at college. He is probably beyond your reach with getting him to start hobbies to do with you. He probably already has hobbies (or girlfriends) this expensive that he wants to pursue. I wouldn't spend that on a kid about to fledge unless I knew he wanted to do it and he was going to stick with it.
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Genesis 49:16-17