Q: iPhone 6 won't "deliver" iMessages

I've been texting a person daily for nearly two months now - I have an iPhone 6 and he has an iPhone 5. We've always been able to send iMessages back and forth without a problem.  Then, the text I sent him Wednesday night never got the word "delivered" under it even though it fully sent, and he never responded to me.  I sent him another text last night (Saturday night) and again it wasn't "delivered" and I never got a response.  I can send an iMessage to my friend with an iPhone 4 and it will be delivered though.  Any ideas why this is suddenly happening?  I was under the impression that when someone blocks your number it's a lot less obvious.