It's not a problem being vague, it's just that I can't make a good guess based on it, and if I knew the details then I might say something very different, so I pointed it out as a caveat.I must admit I'm curious how you challenge each other online. I know it can happen, but will it still happen offline? People can have very different chemistry online than they do offline, interesting enough.
It's also worth noting that your glowing praise of him seems based entirely from online interaction, something to be wary of. And a player or pick up artist would say many of the same things (it doesn't mean he is playing you, but just because he knows what to say to you doesn't mean he's presenting himself and his intentions honestly).
IF he is being authentic with you, and your praise of him is justified...and that's a big "if"...then perhaps it's time for a change for you all (and hope the chase and longing isn't much more exciting than the actual having).
Change happens to all of us, and that includes both love and life arrangements. People can fall out of love as much as fall into it, and there isn't always a clear reason why it happens, and may be as seemingly random as falling in love. Sometimes 2 people can fall out of love with each other yet still love each other in a different way.
You have romantic views of marriage, so I think it's worth pointing out that marriage varies a lot more in actual practice than many realize, and the idea that marriage is about One True Love is a relatively recent notion. It is based on property rights and paternity, and has been used to seal deals and treaties as well as create alliances and mergers for thousands of years (a woman rarely got to choose who her husband was, though some cultures did allow a woman to declare divorce if it suited her, but her family would be a strong factor in whether she did or not), and had to take many things into account (from race, class, religion, and many other things). Of course, marriages could have more than 2 spouses (that is each other), and in some cultures a woman can have multiple husbands just as a man can have multiple wives, and some places today even have etiquette about lovers (for the wife as well as the husband) outside of marriage (it's expected, within boundaries).
And until recently marriage was seen as different from love, and sometimes even sex (especially at the higher levels where marriages were purely political, typically foisted upon the man as much as the woman, and beyond offspring they were both assumed to have genuine lovers outside the marriage--sometimes a husband even financially supported his wife's lover, just as he did his own mistresses--and often his mistress's children). I know plenty of spouses tolerate sexual partners outside the marriage even now, though of course most would rather hide such things given the legal vulnerabilities it usually creates, not to mention the social stigma. That's because the institution of marriage changes over the centuries as much as anything else, though human nature tends to stay about the same.
If you both feel marriage is primarily based on feelings of love then by all means this man should drop his wife whom he no longer loves (at least in that way, but sounds like there is no respect there either) and get with you. He has likely changed over the years, but at least he should have a much better idea on what he wants, and will want in the years ahead...perhaps you will even marry him, and should his marriage to you end then maybe it would be you who realizes you want something else. Feelings are mutable things, after all, and people grow and change (which can change how they feel about a person, even a spouse), and it happens quite a bit even as they reach mid-life and beyond.
Speaking of which, there are women who prefer being the "other woman" as a mistress is generally treated with more appreciation and consideration than a wife. A wife has her own advantages, but it's mostly in terms of legal issues in society (assumed inheritance, insurance, tax purposes, medical and other visitation rights, etc) whereas a mistress (or a male counterpart) will get a lot more heated passion, and probably a lot more gifts (and fewer obligations) than a spouse gets. It's just something to keep in mind as once he actually has you then his behavior toward you may start to change.
Note, I'm not making any moral judgments here as I don't enough about the specifics, I'm simply sharing my thoughts as you seem to want as many thoughts as you can get in figuring this out. That's all I'm doing, sharing things that you might find useful to keep in mind. I currently wish all of you the best, and it could be that it's in everyone's best interests (including his current wife and children) that he ends his current marriage, and if so, then full speed ahead.