Thanks for helping me with this, everyone.

I have read both Gavin De Beker books by the way, just in the past few weeks.
Well, I have a little more time to think about this now because SIL, MIL, and DN are coming out here ahead of FIL now and he is staying back in IL for a bit longer to settle things up with the house, etc. So that is one good thing. Maybe he won't come at all! (I can hope!)
I really don't know the situation with DN and FIL now since we haven't been around them in almost a year, since we moved here. Soon I will know a lot more, and be able to see DN on a regular basis.
Two things that keep sticking in my head since reading this are that-
It could have been *both* FIL and bio-dad that abused DN. I never thought of that before, but it makes sense all around.

Also, that everyone knows what FIL did and how I feel about it... So it's not like me enforcing boundaries there is going to come as a huge shock or something- maybe it will be easier than I think.
Right now, DH and I really need to talk about it more. It is just hard with him working sooo much, but I will pin him down before FIL arrives. I know whenever I make a big decision that DH doesn't like, in the end I am usually sorry. (I hate that, but it's true.) I kind of think, even if DH is wrong, and FIL is a molester, FIL would never come near my DC with a 20-foot pole, because he knows I am watching him like a hawk, and I think he is a bit intimidated by me since the night I let him have it and printed out the porn for MIL to see, etc. He knows I mean bussiness, am not afraid to publicly call him out, and to do anything to bring attention to the situation. It's not a nice thing to say, but the guy has a lot easier targets than *my* DC at this point. *Anyone* would be a better target, and I think he would realize that.
As for whether it was kiddie-porn, I don't think so. I have seen his porn since the time I was in high school, and the 'young' is just the gross barely-legal girls that look really young. The only good thing about the internet is now his porn is online and 'hidden' and not laying around, yk? Still, that is one reason why we really just have always invited them to our house, and don't go to theirs much at all. I am sure that will stay the same here, and FIL won't want to come to my house of all places...
Anyway, right now I am just considering compromising with DH and letting FIL be around on big occasions in more public places (I'm thinking like 1 or 2x a year in a park or restuarant or something), and flat out telling DC to stay away from him, and having DH pick up the slack by being attached to his dad's hip, while I do the mommy-thing. Or, maybe I will get lucky and FIL will be more than happy to aviod us and just stay home.