8 month old yellow Lab dominates 2yr old black lab

We adopted Rex back in March to be a companion to Bo, our 2 yr old Lab. Bo had an older sister prior to this, but she ped last Nov. So, Bo is use to having another dog around. Abby was the dominate alpha dog. Now that Bo is suppose to be the alpha dog, Rex has totally taken over Bo. Its as if Bo is afraid of Rex! It wasn't like that at first when we brought Rex home, but now that he has mively grown, the roles have seemed to change. We have separated them during feedings and they have separate sleeping areas. Bo will not go outside if Rex is out there because he gets attacked! I know Rex is playing and once in a while, Bo will play, but usually quits after a couple of minutes because of Rex's aggressiveness. We walk the puppy and play to tire him out. We walk Bo as well, just not at the same time! I feel bad for Bo because he will just stay upstairs if he knows the puppy is out of his crate. Are there any suggestions out there?