He is probably very shy and really doesn't know what to do or say. It could be helpful to do things with other couples so he can learn some by copying what other guys are doing with their girls. These first months in first relationships are very awkward, and boys usually have a lot less experience than girls, and they also DON'T talk to each other about how to act with girls. He probably needs somebody to talk to him about what to do. fiddling with his phone is a way to do something he knows how to do. He's NOT blowing you off; he's just scared that he'll do something wrong, and you're nervous that he doesn't like you, because you need his approval--and he doesn't know how to show it. Draw him out to talk about things that he likes to do, even if you're not very interested in those things. That will make him more comfortable if you act interested in what he likes. You can also tell him about things that you like, and maybe he'll be able to show you interest because you have shown him interest.
I've read dozens of relationship histories of Jr & Sr HS boys who fled from relationships after the first month or so because they didn't know how to act at all. One wrote "I was supposed to act like a boyfriend, but I never got the memo on how." Go to activities like bowling or skating where he can do what other guys in couples are doing.