I know i'm a little late with suggestions but you could do alternate ending.
So let's suppose the dad starts to treat his son like shit and making him worship him and his muscles. Then the son comes into contact with the thing that is making his dad grow out of control and he gradually gets 2 times as big as his dad.
Ex: "hey dad how are you" I said on the phone. "I have a *giant* surprise outside for you. I won't fit through the door so can you come out I want to show you." I said arrogantly.
"Sure whatever puny." He teased.
"Whatever you say dad."
He walked outside and his jaw is dragging on the floor.
"Y-you were smaller about 5 hours ago. Like a lot smaller. I was double your size."
"Well after you treated me like absolute dog shit. i though some payback was in need."
"You used me for things I could not fight you for because you were so massive. You made me worship you and your muscles, towel you down after showers, You forced me into sex when you couldn't keep your horny body away, but finally the worst thing you did was make me feel like I wasn't your son. You made me feel as I was a toy and object used for your bidding. Kids teased me as I rode my dad's cock into school since you couldn't drive anymore."
"Son... Think about your actions before you do them."
"You're one to talk after all you've done to me."
"I apologize for the things I've done. The muscle was talking."
"Now before I split you in half with my dick the size of you. I would like you say a few words and then maybe i'll let you go."
"Son... I just wish I could've been like you ,but my muscle made me lust for more of them and more. I just want to spend time with me son. I know I've treated you badly ,but know I actually woke up blacked out after our sessions of intercourse.
"Okay now. I feel as you've made a strong platform ,but Its my turn to do something."
I held my dad up and we fucked till he had came and came.
He toweled me off for hours as he is a bit smaller than I am.
I then told him everything would be fine if he would never treat me like that again.
"SON where did you learn how to fuck like that no one I've been married to has done that good a job."
"While you were living at the gym a few boys noticed my growth and hopped onto my train and I practiced and practiced."
We grew till my dad had passed away at the ripe old age of 98.
I had to pay for an entire cemetery as he was massive ,but I had now dwarfed him. So barring him was easy