Well, what does he like to do? It's hard to suggest something since we don't know him. If he's from Deux-Montagnes he's probably already aware of the festivals and has done them, so I won't suggest that.
Basically, I would say the idea is to show him that he can have the life he wants within the city and there is a way to reconcile your two visions of the future.
Have you taken him to hideaways within the city? like bars or restaurants with back terrasses - they're quieter. Spend one night out a week doing that (not free, but Casa del Popolo - when there's no band- , Ste Elizabeth and Santropol are all reasonably priced and mid-week they're less crowded).
Show him the world of cheap and accessible nightlife and food. Have a date night where you go out to dinner and explore all the restaurants.
Get him used to going for walks on the mountain. Maybe not on Sundays, as the hippies may scare him, but during the week it should be good. I assume if he's from the 'burbs he's probably outdoorsy and may miss nature and quiet - show him that it's possible to have it in the center of the city. It's too late now, but you can do cross country skiing and snowshoeing up there in the winter. Nevermind the other parks, like Lafontaine and Westmount.
Take him on adventure walks to explore other neighbourhoods. the Plateau is great, but it IS noisy. I live here too and 3am is a pain for everyone. Other neighbourhoods like Cote des Neiges, Outremont/Mile End (and further up along Bernard) are also vibrant and more quiet - and may inspire him to want to stay urban!
Especially NDG. In fact, as you're aware, this is probably the neighbourhood that will nab him, since its residential and cheap and a lot of suburbanites really like it (though I personally think its dead and no fun) . Take him for a walk in Monkland village with ice cream. If ever you were considering a move out of of the plateau (assuming you still live here), maybe you guys could do a test run there to see how he likes it!
Get him into cheap yoga at Moksha, there are communtiy classes where you pay a really small amount (I think it's by donation). Get him a Bixi pass, if he doesn't have a bike, and you guys can take bike adventures in the evening, maybe along the lachine canal or eastward or anywhere along the path (or off!). Have picnics in park Lafontaine.
Is he an intellectual type? Take him to the museum on free night, take him to talks at McGill and Concordia.
Summer is coming up, so there's a lot to explore - the street sales (if just to browse), the street festivals in every neighbourhood, the free shows, the free movies, etc.
That said, I could recommend more specific locations if I knew what he liked.
I 100% agree with you. You're brave for marrying a suburbanite - they're hard to win over, and they may not have the same appreciation for all the things we love about montreal.