The SIM comes with the online order, it's a tri cut card with all 3 sizes in one package, so you push out the size that fits his phone, insert it, and the line goes active. You don't need to buy one ahead.
I'm not aware of a way to enter a SIM you bought separately during the online checkout, so picking one up at a store first wouldn't really save you anything. If you do grab one, make sure it's a postpaid SIM, prepaid and postpaid SIM cards aren't interchangeable.
If you'd rather not wait on the mail, doing the activation in a Koodo store is the faster route, the rep activates the SIM and starts the number transfer right there. Bring valid ID. Porting info here https://www.koodomobile.com/en/help/moving-your-number-koodo