Well, my opinion is that guys tend to go through pornographic phases. They may last years but usually pitter out in the end. It sounds like this guy is into young girls - but that does not make him a paedophile because paedophiles are into single digit females. Take 1-9 years old.
So, that said, it could be an emotional issue for your boyfriend. Perhaps he is attracted to the innocence of youth, or perhaps they make him feel young again since he is getting old and this is his way of coping with death and the idea of growing up.
Also, perhaps these girls are more on his level and he feels less pressure with them because for instance they do not have to deal with paying the bills, doing a 9-5 job, responsibility and stuff. They seem care free which most are I guess. I know when I was 16 my only worry was getting into college.
Anyways, I doubt this is anything to worry about - I feel your fear stems from the fact that you are not that age and perhaps do not feel like you are attractive to him deep down. But rememeber, as you both get older together you will have a history there that binds you together and you will be stronger to deal with this rejection of him looking at virgins.
If you want you could always buy a school girls uniform to dress up for him in the bedroom to show that you are cool with this - like a white shirt, tie, short black skirt, and black tights. Even a bag to carry lol with lollipop
Anyways - like I said only start to worry if you find pictures of pre-teens on the computer because it may come to this and you are right to keep a check on the browser history. Dont stop doing that and if you find any preteens on there talk to him about it. Confront him to seek help for that but only that and not when they are 15-16 cause like in the UK 16 is the legal age of consent last I checked and no one really bothers when a guy dates a young girl of that age only some think that the guy must be wacked if he cannot get a girl his own age.
peace