What do dads do?
They make lousy moms.
It's kind of subtle but I've been on both ends of this now.
One thing that happens to young men as they mature is they discover they can do a lot of things that mom and their older sisters can't do. Like open jars or pin an older sister on the floor so she's completely helpless.
Dad's job is to make sure that the son understands that being physically powerful doesn't mean that you don't try to get your way by bullying everyone, showing by example, the proper way to behave.
And then there's that subtle and unspoken influence. Mainly that dad is still bigger and meaner than the son and the son needs to think carefully about what he says to dad's girlfriend and other children.