I really agree with those saying it would be disrespectful to ignore him. The poor guy put himself out there and you said you had a nice time... You can't just treat him like he ceased to exist! He's a human being!
Yes, his feelings may be a bit hurt if you say the chemistry wasn't there for you, but he's a grown-up, he will deal. Think of it as a test of being a mature adult... For both of you.
I have already said I'm not going to ignore him I said it in the OP
Although personally I don't think its that terrible, if I went on one date and then the guy wasn't interested I would take the hint if I messaged him and I received no reply, eh I wouldn't cry over it or anything.