Well, he's on the young side, but I'd ignore it.
The kid is going to need healthy boundaries when it comes to such things, and you can screw stuff up pretty bad right now if you start intruding or giving him guilt complexes or something. The kid's a warm-blooded American male and has a pulse. If you make a stink about that, he's just going to get even more curious and want to figure out what else is out there. I'd let it be. I'm sure a number of females might throw a fit, but every healthy male in their early to mid teens is one way or another going to have a Playboy or two stashed somewhere.
In addition, by ignoring it now, you'll likely be able to know in the future if he starts finding other stuff that's a lot worse. Make a fuss and he'll just find much better hiding places.
I'd figure out what your main concerns are and why on your own, and address them at another time and place. If you think he's seriously dating (or wants to), if he needs a generic birds and the bees talk, etc., you can bring those up without referencing the magazine. If he seems to start treating girls/women differently, hopefully you'll notice and can maybe understand a potential reason why and deal with it then. You might want to keep an eye out for how you think he may have gotten his hands on it, too. Though most kids can snag something from someone else's older brother, etc., etc., you don't want some lecherous adult/teacher somewhere being the one to give such things to him.
Anyway, I'd personally more-or-less ignore it for now, sit back, and essentially monitor the situation until you can figure out if there's any problem or simply something where you need to let a "healthy" boundary develop. I would try and make sure he doesn't go after harder stuff on the internet or start getting involved in (online) relations he shouldn't, however. Watch the internet use more than anything. Watch if there's any sudden change in who his friends are, his behavior, etc. I'd try to be a parent he can come to if/when he has questions or needs help more than anything else, and I do think that means sitting back and being quiet for now.