I need advice PLEASE!

I am really torn and honestly just need to vent and have some people to talk to.

Last night, I was at home with my boyfriend, his son, and our daughter. His son is 8 with severe behavior problems. He started throwing a tantrum because he wouldn’t clean and my boyfriend turned off the tv.

My boyfriend tried talking to him and started to lose his temper. He says to his son “straighten up or I’m going to punch you.” I was a little taken back. I tried to stay in different rooms during this because I don’t want our daughter around it. They get pretty crazy when fighting. Well my boyfriend ended up calling his son a few names then took him downstairs, threw his LEGO creation against the wall, and smacked his son’s leg to the point it welted and still was visible this morning. His son spent 1+ hours screaming his head off before calming down. I tried to de-escalate the situation, but couldn’t do much as I was protecting the baby.

I don’t know what to do. We have tried family therapy before and individual, but they don’t stick with it. I don’t want my daughter to grow up exposed to all of that and I’m growing resentful because his son’s behavior and his own temper is pushing me to the edge.

I don’t know what to do. It’s been 5 years of trying to make things work and I’m afraid they’re only getting worse. I feel like this is abuse and I just can’t take it anymore. What should I do??