No offense taken. Since this relationship started two years ago, I’ve had to come up with ways to explain it, not only to friends and family but myself as well. (And for what it’s worth, my SO is 29, and I will be too next weekend.)
In the case of relationships like these, you need to discard the idea that there is one way people fall in love-- that is, by meeting in person, talking a lot, and then falling in love. That first step gets pushed to the end. Which is no big deal, because assuming the people involved are being honest and sincere, there’s no reason two people can’t fall in love without ever seeing each other in person.
The assumption of honesty and sincerity is a big deal, though, and it should never be assumed that the person you’re involved with is presenting themselves as they really are. But it’s up to the people involved to develop their own bullshit detectors. Trust has to be built. In my case, in addition to conversations my SO and I had, I took a look at how he treated other people, and how other people treated him. I got to be friends with his real-life friends, and met them in person last year. Nothing I observed or heard about him contradicted with the mental construct I had of my SO, and that added to my trust in him.
The online aspect aside, our relationship isn’t very different from relationships we’ve had that started face-to-face. How did we start talking? By seeing that we had common interests. (We met playing a MMORPG and chose names from the same fantasy book series.) How did we get to know each other? By talking a lot-- making jokes, talking about our lives, talking about ourselves. How did we develop feelings for each other? By liking the person we were getting to know. How do we sustain the relationship? By doing the things anyone does to sustain any romantic relationship.
You said that one way you know you’re in love with someone is by seeing them and feeling the spark of physical attraction. I agree, and it’s one reason I’m dying to meet him in a few weeks. But physical attraction, IMO, feeds infatuation more than it does love. I don’t mean that physical attraction is unnecessary-- it’s definitely important to be physically attracted to your SO. It just isn’t a strong indicator of love to me. I’ve had some pretty shallow relationships where physical attration was clearly there.
If my boy and I meet and there’s no spark there, then at least we tried. Two years ago, we knew we’d be jumping into a long-distance online relationship. We decided to go for it anyway. We figured at worst, we’d have a great friendship and the memory of a very sweet romance. That’s a lot better than what other people are left with.
None of this is to say you have to give your friend your blessing. In fact, I think you should help him keep his feet on the ground, and encourage him to carefully think through what he’s getting himself into. Don’t rain on his parade or anything 
but try to keep him from getting carried away.