How closely was he shaved? A longer trim is less of an issue, but shave close to the skin will mean he has a while for his coat to recover.
In many cases, the coat will grow back. It'll take time, probably a few months, and you'll need to be brushing him out daily (with something gentle like a regular pin brush or slicker, not a deshedding tool) to make sure you're removing dead undercoat and giving his longer, slower growing guard hairs time to grow out properly. It'll likely look a little strange for a while, as you'll be seeing more of that soft, fluffy undercoat than the longer, straighter topcoat you're used to, but it's unlikely to completely destroy his coat.
If you do notice his coat isn't recovering after 6+ months, you might want to get some bloodwork just in case, as this can occasionally be a sign of an underlying issue like a thyroid problem.
There is the rare dog whose coat never comes back right from a shave but have no underlying health issues. It sucks and I hope it doesn't happen to you, but it's definitely a less common thing than a lot of the "never ever shave a double coated dog!!" articles online would have you believe, so there's a lot of hope for your pup!
And if you have to use a jacket for a while, go for it. My poodle has a terrible coat for his breed - very limp and soft and doesn't protect against cold or wet much at all - so he's had to wear a jacket in very cold or very raw, wet weather his whole life. If you get a quality brand and take good care of it, they have decent resale value too, so you may be able to get some money back on a site like Craigslist (not sure what you use most in Canada) after your pup's hair has recovered. We like Hurtta brand here - they're designed in Finland for active dogs doing outdoorsy activities year round so they know intense winters! They're on the pricy side though, so an inexpensive jacket that you can try on at a local pet store before you buy might work just as well for you.