
is he usually bad with texting or communicating when you aren't around each other? not trying to defend him but i tend to be horrible at texting and responding to texts when i'm busy with anything in my life. if that's a dealbreaker for you, don't stay and wait to be resentful enough that you can't stand it- do yourself a favor and leave. if it's abnormal for him, i'd talk to him when you get back and ask if everything is alright in his life, or what caused this change.
if you don't trust his answer, then it honestly isn't even worth looking at his phone because the trust is not there anyways. just break up then. but consider whether this behavior is out of character, and if not, whether you can be with the type of person who isn't good at digital communication when apart from each other. if you can't do it, don't.