The answer to your question was already provided in the story:

WE NEED MORE SUPPLY

So if you want to alleviate the problem, enact policies that encourage developers to build rental units.  Things that help this would be, for example, clear, simple and straight-forward zoning rules.  Things that will make things worse are policies that discourage building rental units, such as rent control or forcing developers to compete with government-built units that can (will) be rented at a loss subsidized by taxpayers.  That will discourage developers and making them take their (very mobile) capital elsewhere.

Why bother building in Ontario where you can't make any profit when you could just as easily invest in a REIT in the U.S., Europe or Asia.

... or simply move to the U.S., Europe or Asia and build there.