He is able to perform essentially a citizens arrest I believe but for that to be legal he would need to contact the police (I assume Calgary so CPS) ASAP. Past that the amount of force he would be able to use would be the minimum required I believe and in this situation it sounds like anything but that was used.
The more important issue is him putting himself at risk in my opinion though, not of legal ramifications, but rather would you be willing to get stabbed or worse for a couple magazines and a pop. I know that it sucks to have anything stolen, but the financial and emotional costs of being attacked by someone are much worse.
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