I wouldn’t stress about it, ALL men watch porn, whether us women like it or not. Some men like to watch it with us, some men like to do it privately on their own. It could be his time, when he needs his own space and so therefore he watches porn. I understand it bothers you, but how do you know he stays checking so often, are you asking him, or do you stay checking his phone? You could be the sexiest woman in the world and he will still watch porn. Better him watching porn rather then talking/flirting to other women or worst even meeting them. Maybe you could try watching some porn in your own time too, and try to understand why he likes to watch it alone. If it gets to the point that he checks several times a day he could have an addiction, which you can read about online to get further info if you wanted to research further.