Think you're over-reacting. Unless the message is overly important to reply to, he's under no time limit or obligation to respond.
Honestly, I don't live my life by my phone, and having to constantly engage in a stream of messages when I have to work or am even just chilling is just time-consuming and taxing. A quick conversation, every now and then, I don't mind, if it's of importance and interest/funny to both of us. However, having to engage in messaging for 10 mins or more just isn't appealing to everyone. Some people aren't beholden to constantly chatting on their phones.