Clingy LDR

I'm really an independent person, and my SO had previously appeared that way too. Lately, however he's been annoyingly clingy giving me a hard time that I'm talking to other guys during the time I don't text him or he complains about how often he checks his phone and he doesn't get a text from me. I'm trying to work with him and tell him when I'm busy and can't talk but I'm really not consistent with it and don't feel like I should have to tell him when that I won't be responding within 5 mins. I'm not even back in school yet but it's getting worse and worse. I'm worried it might be the start of a controlling relationship. How should I approach this?