You and your son will have to go over to your nearest Koodo kiosk with his phone. They will ask you for ID to confirm you are in charge of that account. Then, you will be able to add your son as a second line on the same account. He can even port his number to Koodo should he want to keep the same number.
Do make sure that his phone is unlocked if it isn't already or a Telus phone. Since you mentioned android it's best to use a third party such as sellers on ebay. They will provide instructions what how to do it.