When you ported out your number you effectively closed your Koodo account. You can still use the phone with Koodo, but you will need to go activate a new account in a kiosk. You will also need to buy a new SIM card at a cost of 10$ upon activation.
Given that you could not keep a grandfathered plan the moment you port out, the only option is to add a new line with Koodo. Had you done this prior to porting out, you could have kept the same account number. The only thing this affects is billing.
If your son is the age of majority in his province of residence, he can get the line to his name and take advantage of Koodo's amazing new activation promos. Otherwise, he can go with pre-paid, or you could activate a line for him to your name (and absorb all liability that comes with it).