I find it hilarious how many weebs like myself are on here talking about it being easy to tell a girl she looks good. It's obvious the senpai is an adult with zero useful exp with women. He's super shy and probably still can't believe he's going out with his dream girl. I'm not surprised he's acting this way.
Remember that until a few chapter ago Douki-chan was behaving this way. She's just relieved now that she bagged her man. He on the other hand is still coming to terms with it