As I see them, the issues are these: on the one hand, I doubt that I can completely control his access to visual material so he will doubtless find something even if I forbid it, and in this situation I will have no control at all and won't know what he's looking at.
On the other hand, I certainly do not want to appear to be implying to him that I approve of the objectification of women (sex as object, rather than part of love) by approving more placid pornography.
My sense is that he's certainly going to masturbate as much as he wants (and neither my wife or I want to make him feel guilty about it) but that he will, somehow, seek out the materials he wants - so it's better to guide him rather than refuse complete access to porn. You're obviously a parent who cares a lot, although your question is a shocking one.
I think you're right to cut him off from Internet porn.
If you want to guide him a little, why not buy some tame magazines as if they're your own and leave them where he's apt to find them?
I think 12 is definitely too young to be an active consumer of girlie magazines.
I recently came upon my 11 year old daughter's diary and I found out she masturbates and it disturbed me a little.
But when she came home and I spoke to her about it, she was ok with it. I have a girl here who masturbates daily during nap times.
Now, I have absolutely no problem with his masturbating, so I think I'd like to guide him toward less extreme visual material. Should I buy him relatively innocuous magazines like Playboy - or even let him pick out what he likes (with guidance, of course)?
I haven't broached the issue with him as yet (I'm sure it will be difficult for us both) but he seems to be a lot more intensely interested in porn than I was at his age, so my own experience may not be that helpful in judging what's right for him.