If he was playing he was obviously not staring at his phone waiting for text messages, unless you wanted him to text you saying "I'll be gaming for a few hours won't be near my phone" before he started playing?
Btw, idk anyone who does that.
If everyone got upset with me for not replying "on time" I would have no friends or bf. I don't have a job where I can use my phone freely and sometimes I just want alone time and ignore it for a while.
In my opinion, the issue isn't him taking an hour to reply, instead, you should rather look at why the delay upset you to such an extent.
No doubt the boy is interested in you, he's made that clear, initiated your conversations and so forth, so I would say you can definitely cut him some slack.
Not everyone's phones are glued to their hands.