How do I deal with him?

Please help.  I am dating a guy for almost 2yrs now, But his 11yr old son is driving me up the walls.  My boyfriend gets the children every second weekend and when they are there the boy will do everything to drive me up the walls.  This past weekend they were there and we had a braai at my parents where my 3 boys stay.  My youngest son (8) has a hart condition and saterday my boyfriends son hit my son full on the chest on his hart.  My son fell to his knees and gasping for air.  My boyfriend went and gave his son a quick hiding.  Sunday when we took his children home his boy told the mother that his father gave him a hiding for nothing and she confronted his father about it without listening to what happend.  My boyfriends son his lying to him and his mother and the child lied to my boyfriend several time that caused trouble in my relation ship.  Please help  me on how to treat his son and what to do? before I break up my relationship over his son.

[specialmom]

Honest opinion, find a new boyfriend.  He's got a responsibility to the kids first and it sounds like it is going to be really hard.  His children have had a lot of change in their life and it is hard to go from one house to another with different people and rules.  It is just how it is with kids.  He may also not have been taught (which would be his mother and father's fault) the right way to behave.  Kids don't just "know".  They need good role models (and hiding isn't really the best way to teach.  It gets your point across really fast but what lesson did he learn?  I hit and dad hits.)  They need structure and adults that have taken the time to patiently teach them what to do.  It's really hard, I know.  And this is a situation in which his kids have him and their mom and it is out of your control for how they act.  So, I'd find a different more pleasant situation to be in.  Do your kids live with you?