as far as the "hmmm" thing is concerned, I think it depends on the context. I'll say "hmmm" when I'm being flirty. I'll say something like, "you really need to find something else to do on the weekends. Something other than same old boring stuff, hmmmmm" kind of like "wonder what you could get into, maybe me"? But I wont say it I'll just say "hmmm". That's me thinkin' naughty but not saying what I'm thinkin'. Make sense? Or like you looked super cute today, hmmmm". It sounds like you're just a little insecure about the relationship which is normal especially at the beginning.
The part where he doesn't text you back. Sometimes it could just be an honest oversight. Does he have AS? This might explain things. Sometimes I'll text and get nothing but it's because he's thinking really hard on how to respond and then he just burns himself out on what to say so he just doesn't respond at all.