You might be able to guess that I’m a big fan of Hoosiers too. I spent some time with my little cousin when she was about 4, and she was learning to play (You should see her now, incidentally-she can dribble behind her back, and she’s 6!). The only thing I would recommend is that you help Julian get started, and then leave him to his own devices.
Most likely, all he’ll care about for a while is shooting the ball. At the beginning, he’ll need you to show him how to bend his knees and shoot one handed, using his other hand as a guide and not actually shooting with that hand. If he has to fling the ball, he’s too far away. If he’s hitting the bottom of the rim, he’s too close. Anywhere in between is good.
When he shoots, tell him to look at the front of the rim, and try to nudge the ball right over that spot. Teach him how important follow-through is on his shot. Any shot that hits the rim or backboard is a successful one.
Teach him that you have to dribble, and how to dribble, with only one hand at a time. Watch some games with him if he wants, and he’ll probably want to do what they do. Help him with that.
Beyond the basics, though, I think he’d be best off if you allowed him to go at his own pace. Play with him when he asks you to, but that’s it. I think a lower hoop and a smaller ball is a really good idea, because if he can’t get the ball in the hoop at least a few times, he’ll get frustrated real quick. Once he understands how to play, it’s pretty much up to him if he wants to take it from there.
Above all, don’t try to teach him anything he doesn’t seem willing to learn. Trying to run a play, etc. is a good idea eventually, but it probably won’t be fun for him. If it’s not fun, and he hasn’t become attached to the game, that will turn him off to basketball just like that. Most likely, he’ll either take to the game right away, or not at all. If he does, you won’t be able to tear him away from the basket, and then you can work with him on anything he wants to. In my case, my love of the game started at about that age, and I was content to throw the ball up at the rim for hours on end. I’m not sure, however, that I’d have enjoyed learning anything more advanced at that particular time.