i personally think it's quite possible to fall in love with someone via the internet even without having met them. There is so much that you can learn about people through talking, chatting and such. In fact in some ways those things are far deeper than what you get to know by dating someone too. many times in real life dating deeper subjects aren't even covered in the beginning because the physical attraction could overshadow these things and you're too busy being all giddy about it.

My thoughts based on your short post (and I haven't read all the replies, btw) is that if it's been this long he's been talking to you and now he is just saying he has fallen for you, I see this as quite possible and probably more valid than you think. It's not like he's faling for someone that he's talked to a week but it's been what around 2 years now? I don't think you should find that so shocking. He probably knows quite a bit about you by now.

I wouldn't brush this off at all. I don't know how you feel about it but in any case, it could be very real for him.

Just my 2c.