My 2.5 yo son keeps biting and hitting my 4.5 yo daughter.

I'm 8 mos pregnant, and have been put on light duty for preterm labor. I know his triggers and I can tell when he's going to snap, but sometimes I need to leave the room. He'll bite/hit, she cries, then I don't know what to do. I tell him we don't hurt other people on purpose and ask him to move away from her. But then what? He just keeps doing it.

What am I going to do when I'm nursing a baby, changing a diaper, etc and he starts hurting her? I can't just let him abuse her, but I'm not sure how to protect her and teach him that its wrong.

Please help.