You already asked this, and responded even, so it's not like you didn't see the comments people gave the last time. So it seems like you're fishing for a specific answer in order to justify what you want to do, which kind of begs the question of why you need anyone else's approval to do something nice for your son either now or at some point in the near future.
Just do whatever it is you want to do, and rationalize it to yourself as quite simply doing something nice for your son. You don't need a bunch of other people telling you it's a good idea. Who cares what we think? You shouldn't. There are plenty of kids who will be in the same classes as your son who would wish their parents would get them a laptop either when they started school or as a graduation gift.
That all said, I would wait until AT LEAST his second year. First off, your first two years of college are typically just repeating high school at an accelerated rate. It's full of a bunch of gen ed crap that you have to take to get the requisite GPA and credits needed to take the degree courses. So he probably won't need a laptop at all for those, let alone some high end thing. Second, there's always the possibility that he will get there and find that graphics design isn't for him, or maybe some other major just kind of calls to him. There's even the possibility that he will decide college itself isn't for him, or maybe that specific school isn't a good fit for him, and then he goes some place else which emphasizes the PC version of programs instead of Mac... There are just so many different ways things can go sideways during that first 1-2 years of college that I'd consider it rather foolish to buy anything until you're reasonably certain things are going to go the way you think they are. If by the end of the second year he's still committed to the idea of graphics design, and at whatever specific school he's planning on going to right now, you can look into laptops then.
By that point I would expect that even Apple's fairly deep product development pipeline has started to run dry of things that Steve Jobs had a hand in, and Apple is well into its inevitable backwards slide. Especially with the way Apple has been kicking the graphics market in the teeth of late. First there was the Final Cut Pro X brouhaha, the extremely long wait for a Mac Pro refresh, and then to call what they finally did release a letdown would be a serious understatement. Adobe's relationship with Apple has been rather frosty at best ever since Steve Jobs decided to ban Flash from the iPhone. Don't get me wrong, I'm one of the biggest flash haters around, but to have the CEO of Apple running around publicly badmouthing one of your flagship products doesn't exactly make for a good working relationship, now does it? So it may very well be Apple's fortunes will be on the wane again, and the wind is blowing in the Windows direction once more, making this whole discussion of which Apple laptop to buy rather academic (pardon the pun). Then again, maybe it'll be stronger than ever, and you'll be able to take advantage of one or two additional refreshes of the hardware in the interim. Either way, your best strategy would be to wait.