Who said he's unhappy? He's doing this because he's a slag, that's all there is to it. Open and shut case.
Plenty of happily married men cheat on their wives but have zero feelings for the women they cheat with and absolutely no intention of ever leaving the wife.
It'll be a quick shag, a few If you're lucky, and then he'll do the slow fade whilst you drive yourself insane looking for hidden meaning in the text exchanges you had and convincing yourself that he was just so into you but can't be with you because of the kids, dog, house, etc.
You will romanticise and rewrite the whole thing into some star crossed lovers situation and become increasingly bitter towards his wife who is obviously the problem in all of this.
Eventually you will reach out to her, probably anonymously, and tell her about the affair. He'll tell her your mental or if bang to rights he'll cry and beg and tell her it meant nothing, which will be true.
She will stay with him, probably because of a sunk cost fallacy and the kids, dog, house..
He will go completely NC with you if he hadn't already and you'll spend months in a state of utter heartbreak as you try to get over him - still resenting the wife and convinced that if she'd have just fucked off or not threatened to "take the kids" he'd have left her in the first place and would be with you by now.
Meanwhile in his world - they work on their marriage and probably have some couples counselling then before long the marriage is back on track. Eventually, another gullible (presumably) young woman catches his eye and the cycle begins again.
He wouldn't come back to you though because you'd already served your purpose and proven yourself to be too much hard work, he only wanted a shag.. not an emotional woman pecking his head trying to get him to leave his wife.
Your self esteem will be crushed and you'll struggle to trust men because after all, you've seen how the "good ones" behave haven't you?
You have nothing to gain from this and everything to lose. Block, delete, move on.