how's that? If I feel inspired, I may come back and link-ify, but you know how to use Google and Amazon search, right?
Hmm... he likes the Simpsons. I'm guessing he likes the humor and the drama. If he's only about the sarcastic pop-cult jokes, I don't really know where to send you, outside of some stuff that's really *way* too old for him (e.g. The Exterminators is an R+-rated comic that is a sort of Six Feet Under except bug-spray instead of embalming fluid).
If he likes the drama and goofy characters, see if he likes Jeff Smith's Bone, as blackwingedboy suggested, or Carl Barks' Uncle Scrooge. (I've even heard Bone described -- by its author! -- as "Uncle Scrooge meets Tolkien").
Bone is accessible at about 8 years old, and I've met some precocious 6-year-olds who could probably handle it. (Bone doesn't talk down to its readers, though: it's really a great story for grownups too). A "young 13" should have no trouble with it. Uncle Scrooge is a bit younger, but not much, with a little more talking-down to readers.
I'd hope that one of those two would allow you to lead him away from the butt jokes. If that's what he wants, then Simpsons is probably the place to stay. But if not:
If he goes for Uncle Scrooge, he might like the more modern adventure stuff, so I'd send him to the Tintin books next. They're all pretty great, but I'd skip "Tintin in the Congo", not that it's easy to find any more -- its racism is really awful (remember: mid-20thc. Belgian writer).
If he goes for Bone, then next to try would be Elfquest, if he likes the fantasy stuff, or perhaps the Akiko graphic novels for SF adventure goofiness, if the goofy is what he likes (Akiko by Mark Crilley: note he's also written a series of Akiko young-readers books, but the graphic novels are *much* more fun, caveat emptor).
Elfquest is a bit more of an "old thirteen" than a young one, and there's no cow-races or goofy Bone cousins, which is the bait Bone offers to drag a Simpsons fan into epic fantasy. (Note: don't bother beyond vol. 4 or so; Elfquest devolves into soap opera).
I've been out for a while (boobirdsfly and I went to La Push last weekend) and I've been very busy with research, and just generally enjoying not being on the internet. So I'm way behind on the live journal; my apologies to you all. writeanya asked me about comics suggestions for a young friend who's just turned "a very young 13". He really likes the Simpsons comics, and she'd like something... else. blackwingedboy has already pointed her at Bone, an excellent choice, and... well, I'm just going to post here and point her here: