Everybody needs some release now and then, and if you and him are not having a great sexual relationship at the moment, more the reason for him to look at porn. It is just visually aided orgasm, nothing else. You cant really control what he is thinking, or looking at for this stimulation, to be honest.
Seems to me you both have weight problems that should be adressed too. Could you sit down and talk to him about this? Could you suggest you together take up some exercise, like walking, or cycling as a family? Could you join a dance class together? Could you look into healthy eating and see if there is a way of coming up with healthy and nutritious family meals?
My husband bought me a bike for mothersday last year, the best thing he could have done. After I had the kids, I too had put on lots of weight, had no sexdrive, caught my husband wanking to online porn movie, it was a laugh really, he was so ashamed. I told him not to worry, he has normal male urges. But, I have lost a stone the last month, and got my sexdrive back, and on sunday I was able to cycle 18 miles! Weighloss, exercxise, and getting my sexdrive back has made me a new woman. I am not telling you what to do, and stictly speaking you asked about porn, not diet advise, so forgive me if I am overstepping, it is just that you said it was linked.