Most Helpful Guy
When I was your age, that was never a problem. If you wanted to see someone's photos, you would go to his house or he would bring them too you.
Young people put WAY too much emphasis on social media, without having any idea what goes on behind the screen.
But the answer is simple. If he blocked you, then he doesn't want you to see who he is with or what people are saying about him. He doesn't have control over other people, but he does want to control what information you, or someone else gets.
That means he doesn't trust you, or someone. Since you are paranoid about it, you've proven him right. He is concerned that you will cause him grief if you knew what others might post. On the other hand, maybe he doesn't want his other girlfriend to know about you and the way to do that is to block 3rd party post viewing and tags.
My suggestion, you are too young for a real relationship. You have some maturing to do. Break up with this guy and don't even pretend to have a monogamous relationship for another four or five years.
Most Helpful Girl
Because he doesn't want you to see them for some reason.