Hi Aquarius,
OK - bear in mind that...
a) I have a daughter, not a son. (she's 11, btw)
b) I'm not an expert in these matters, this is all just my personal take on things.
c) Being 9 was a VERY long time ago for me!
The world is hugely different from when I first encountered pornography and I hope my thoughts reflect the new reality.
Your son is 9. I don't know what access he has to the internet. My daughter didn't have her own device(s) until she was about the same age. She then had a low spec tablet with all parental controls installed. This progressed to a laptop, and she now has a low end smart phone. In addition, she has a Chrome book from school. So she's very much a connected kid.
What we've tried to do is go down the education route as far as we can, based on the theory that if you do it all by app and blocker, she doesn't learn any strategies or boundaries of her own, ready for the time she has to excercise her own control. There are blockers on the router (note to self, must update that!)
Knowing what I do about the plastic nature of youngsters' minds and the HUGELY addictive nature of porn for some, if not all who stumble across it, I'd try to monitor all his internet access. I'm now going a bit off piste - but I'd have a conversation with him about masturbation, maybe not just yet, but as soon as puberty rears its head. All boys are likely to masturbate, but I'd suggest that you might want to explain it in a parallel to something like thumb sucking - it's something you grow out of. I know that society currently doesn't see it that way, but it might just help him to not end up with a PMO habit.
I wouldn't let him know about your husband's issues with porn. But I definitely would tell him about the fact that some people DO get too involved with porn and ruin aspects of their lives as a result. The other thing, which is something I've discussed with my daughter, is that the portrayal of both men and women in porn is completely unrealistic. I don't know what your family rules on nudity are, but if he sees normal people naked, he might understand a bit more about the reality of the human body.
If you support him in all aspects of his development (and I'm sure you would) particularly areas that are a) important to him and b) are maybe less traditionally "male" hopefully he will develop a more rounded identity and thereby perhaps be less likely to end up fixated by his male ness.
None of this is based on anything more than my own experiences and observations. If anything else comes to mind, I'll pass it on.
Whatever happens, two things to remember - you will do the best you can with the information you have at the time; and you'll always have support here.
Good luck and much love.
Stuart