My son is currently on a $30 Telstra Prepaid Freedom Plus plan where we can use the credit on Google Play for renting movies.
I am currently on the $40 Telstra Pre-Paid Plus plan, no Google Play credit for me.
My son wants to join Optus but I don't want to let go of the Freedom Plus plan (nor pay $30 once he leaves), is it possible that I take up his plan without taking his phone number? Or is there a way to transfer that plan to me?
No - Freedom Plus is a legacy product so once your son takes his number to Optus, (which means he is leaving the Freedom Plus plan), there's no going back.
There's no way for you to transfer your number into his account.