How exactly he does it appears to be unclear, but it involves pricing discrepancies in Canada, where lower-populated provinces like Manitoba and Saskatchewan see much cheaper cell service. Basically, the scheme involves signing up for an account in one of these regions, where Canadian wireless service provider Koodo offers a 5GB monthly data plan for $48, then selling the account to people who live in more populated regions of the country, where the same plan typically costs at least $90.
This loophole has apparently been around for a while, with both of the aforementioned articles pointing to a forum conversation started in 2008 on a site called RedFlagDeals.com. This post showed a $55 monthly plan (available for $49.50 if you sign up with your own device) that offers unlimited calling, texting, and 5GB of data. The forum's moderators, however, posted an update five years after the discussion was started warning that "any discussion regarding getting this deal outside of Manitoba or Saskatchewan will no longer be allowed."
One article also says other people offer similar services on Craigslist.