I'm so sorry.
We've all been there and it's shit.
I'd not contact him. It will only make it worse.
As well as experiencing being hurt, I think we also know when we aren't 'feeling it'.
It's horrible having to tell the other person.
I can see where he's coming from.
He probably thought you were 'okay' and it just needed more time for the stronger feelings to kick in.
I know men and women who've 'given it time' especially if they meet online without any other contact except that.
I think if you contact him it will hurt more. What can he say to help?
Probably stuff along the lines that you are a lovely person, you're pretty, witty, intelligent, good fun etc etc BUT that missing 1% of chemistry isn't there.
Do you really want that rubbed in?
Try to do something to spoil yourself. Buy something as a treat, phone a friend, wallow and cry your eyes out for a few hours, go for a long walk, eat ice cream- whatever.
Then brush yourself down and say it's his loss!
Good luck x