how to get my boyfriend to stop watching porn

My boyfriend (29M) spends a lot of time on his phone. I (19F) thought this was normal but when he fell asleep I looked through his phone because I was worried he was taking to other girls (he refuses to delete snapchat no matter how many times I tell him it makes me uncomfortable). to my surprise his snapchat was pretty tame. it was when I got to his X/ twitter account that made me concerned. He has two accounts he was logged into. I don't use twitter but I did know he had a twitter account, I've never seen the second one before. when I was looking through it he had hundreds of women in his likes. none of these girls looked like me at all. I don't want him watching porn because I know it can cause impatience and distort their views on sex, body image and women, I also think it is a form of emotional cheating. How do I get him to stop?