It depends, whether or not you can trust him to that extent or not. Everyone have their own outlook, you'd know him better. I can't say he's keeping backups or not. Depends on how as a person he is. Like I've a lot of girlfriend but am not inclined to each of them romantically. Maybe I dated them earlier but now they're my friends or close friends and I don't want to hurt them that bad by denying their requests. What wrong they did? Maybe he thinks from this prospective, thing is you need to talk to him and pick out a way that works for both of you.
In our modern society, we often use digital means to shop for a "better deal". Girls do that with Facebook/online dating as well for backups. At your age, it gets harder to find guys who want to commit that are top-shelf. You'll probably have to settle for a guy who isn't as attractive if you want a serious boyfriend/hubby type if that's your goal. This current guy doesn't sound like he's that type.
Men are always in search of companionship. A lot of times women are fickle and leave us for someone "better" or for an old flame which may be what's happening in this situation. Everyone today in relationships are prepped for it to fail because that's how it is in our society today. I wouldn't worry too too much about it, he may have just accepted friend requests from ex's to be just friends. I'd let it play out and see what happens.
I sort of went through the same thing. I got off of Facebook because it was annoying me to see that my boyfriend had a bunch of girls on his page. I later confronted him about it, especially when I noticed that he had some girls on there that he had confessed to have dated before. I asked him why he would even want them on his page if he was dating me? ...his answer was that he didn't want to offend/or hurt anyone's feelings. Then came "why would you even care if you don't even talk to them"? He later understood and deleted them. The other day he had some requests and he just left them as request's. I don't know. I am very blunt with him and told him that it doesn't make any sense to have them on his Facebook if he's moved on. Maybe talking to him nicely. Tell him to put himself in your position and ask him how he would feel if you had a bunch of guys asking to be accepted on your friends list. That may help him snap out of it.