when/how to separate one male baby?

Definitely separate the boy at 21 days, otherwise he'll most likely get his sisters and mom pregnant. You could adopt a male companion for him and they aren't necessarily likely to fight depending on the setup and the males in question. Neutering isn't going to alter behavior, so there's no reason to neuter a male if he's going to live with another male.

If it were up to me, I'd probably keep him alone and able to interact through cage bars (but please put a lid/top on the cage, as a motivated male is quite likely to climb over a simple grid) until he could be neutered. Call your vet to find out when they neuter. It's not necessarily dependant on age as much as it is weight, as far as when a boar can be neutered.