He has such a bad temper and is mean to his son

I'm writing this as I nurse my 10 month old daughter in our bedroom and I am overhearing an interaction with my fiance and his 8 year old son. His son has ODD/ADHD/ and is somewhat on the spectrum

My fiance has NO clue how to get through to him. Right now, he is having his son vacuum and is yelling at him for doing it wrong "no that's not good enough! It needs to be straight. You're doing it wrong!" When I am pretty sure this is is first time vacuuming?!

His son also does not know how to read. Instead of teaching him, my fiance criticizes him saying "you need to learn to read! When I was your age I was so much more ahead." He doesnt have any patience to actually teach him. I can just see the look in his son's eyes when he gets yelled at or put down all the time. We have started having his son more so i am seeing this side of him a lot more which is making me concerned for our daughter.

I try to talk to him and he just doesn't understand what he is doing wrong. I've suggested therapy- that doesnt work either. He says someone needs to hold his son accountable and stop babying him. His mom and the grandparents aren't much help with this either.

I dont know what to do. I feel bad for his son, but I cant do much of anything to change the way his dad treats him. And I dont want my daughter being treated this way when she gets older... HELP?

He also is always telling his son how he is bigger, smarter, and older and that's why he needs to listen to him but he says it in a VERY mean way...