Check out Dana Krempels' article on stasis -- it's got some good information: http://www.bio.miami.edu/hare/ileus.html
The Bene-bac should have instructions on it. Usually 1 g per 10 pounds, so you'd give 1/2 gram. Most important is hydration -- are you sure hes drinking enough? Especially if he's had some diarrhea, he might be getting dehydrated and that could lead to stasis.
I wouldn't give him any oats till his belly gets better -- the starch can lead to gas and bacterial overgrowth. I'd also check his pellets if they're new and see if they smell fresh or moldy. There may be something wrong with them that is upsetting his belly and why he won't eat much of them.
Brush him well each day so he doesn't ingest any of that shedding hair.
If you see any more diarrhea or he doesn't start eating more hay and drinking more, take him to the vet asap. He could have an infection or imbalance in his stomach and in rabbits that can go downhill quickly.