Is he immature?

It seems like a lot has changed since you guys were "together". I guess he feels like it's awkward or he feels that it's now different when he talks to you. There isn't really anything you can do except just keep talking to him and trying to make him open up more and talk to you like how you used to. Maybe you should tell him how you feel, about how you guys don't communicate as well as you guys used to anymore.