When can babies drink regular milk? (Skim, 1%,2%,And whole)

To sum up the story my son is 8 months old, while he was at his grandmas house she gave him 2% milk. I read somewhere you weren’t supposed to do that, so I told her I didn’t want him drinking it until he’s winged off formula. She told me that all her kids and her grandkids have drank it before a year old and that he loves it so he will be fine.. soooo is this okay? He is my first born and she’s acting like I’m stupid for not listening to her. I don’t want him getting sick. So is it okay for an 8 month old to drink it? Or should I tell her she needs to stop feeding it to him?