AITA for telling my mom no to charging my son rent?

Myself 37f and my son 21m live with my mother 64f. We do not pay rent (she owns the house) but I (just me and not my son) do cover about 80% of house bills like electricity, cable, internet. My son has just gotten his CDL and is working kinda locally and is finally getting a paycheck. He has def splurged his first several checks. I honestly have no problem with this. I don’t expect him to help pay bills.

I actually don’t want him to pay bills. I want him to enjoy his 20s and have fun, be smart but enjoy life. I literally don’t mind. Please go have fun and do all the things, I want this for him. Again be smart (as smart as a 21yr old boy can be). I do buy my own groceries and about 20% of the house groceries. My son provides his own groceries as well now that he’s getting paid.

Here is where I might be the AH

My mom thinks, since he’s doing well he should pay rent and I disagree. He’s just getting on his feet. I think it’s important he enjoy his time “rent free” while he still can. I cover more than enough of the bills for the both of us. I told her if she wants to charge him rent then we should reevaluate our whole set up. If we divided everything by 1/3 my mom would have to pay a lot more per month.

She says “we” should keep our agreement the same plus my son pay her rent. I don’t think that’s fair bc I’d be the one paying the “rent” at the end of day.

I would absolutely pay rent but we would have to recalculate the percentage of bills I pay monthly.

So AITA for saying I can’t pay 80% of the bills plus 20% of her groceries if she wants to charge my son rent?