My 2p worth: don't try to plan out your son's life. I know there's a big leap between keeping a car for him and planning out his entire life, but it extrapolates. By all means give/leave him the supra, but don't be upset if he just sells it on. (if you leave him the supra as opposed to give it to him, I doubt you'll mind what he does with it because you'll be brown bread)
Old cars aren't everyone's cup of tea. He might resent a perceived enforced involvement with it. He is not you, and might not have the same interests. It's hard because we're all car nuts, but think about a hobby in which you have no interest at all (e.g. fishing?). I'm sure if someone takes their fishing seriously enough, they can buy rods etc that are worth thousands each. If your dad left you a lot of fishing gear and told you he'd like you to use it, how would you feel?
That probably doesn't help at all.