First things first - I never had a problem with porn. And then I married someone who struggles to control his porn watching, and oh boy have I found some stuff out.
In terms of him stopping, I'm not sure. It may be that you can ask him to stop and he will. It may also be that you can ask him to stop and he'll take this to mean 'I need to hide it better'.
My primary problem with porn is that I think it gives an utterly false impression of female sexuality, and how men should be expected to interact with us. And this is in a culture where we are already objectified, and already presented quite falsely, a lot of the time, in media.
I think for some people porn can become quite a serious problem. Not always, but for some people, and I suspect for more people than anyone realises. I guess I see it like alcohol - some people don't drink at all for various reasons. Some people drink and enjoy it and it's fine. Some people get into serious trouble. The difference though, for me, is that alcohol does have, in some circumstances, a positive social effect, and encourages/lubricates relations with others. After all - a big 'bad sign' with alcohol is drinking alone. But with porn, I feel like it starts from that position - from it being something done alone. Basically, I struggle to think of a positive effect of porn, aside from facilitating orgasms - which could be facilitated in other ways!
Anyway. I also think (and have seen up close) that porn use really can damage people's ability to have healthy sexual relationships in quite an insidious and severe way. There are some pretty interesting places online to look into it - reddit is a good place, there is a 'pornfree' board where men who have quit porn talk about it, and their discoveries are quite surprising, in terms of the effect that quitting porn has on them. Yourbrainonporn is also a good resource.
To stop rambling - if you want him to stop, I would discuss it with him, in as non-judgemental way as you can. Don't minimise your feelings about it, they're important, but I think a lot of men, unfortunately, have porn-watching much more ingrained as a habit than they even think they do - and they find it quite hard to even consider stopping. It's sad, really, but I guess given that it's such a 'normal' thing to do these days, not really surprising. Treat it as an experiment, perhaps - just to see what effect giving up porn has on him. He might actually like it! I know I've seen quite massive positive changes.
oops, sorry, megapost.