How can I stop my dominant cat from terrorizing the other? (Read below)?

I have two indoor male cats, Charlie and Rigby. I've had them since they were born about two years ago (they are brothers). They were once 'friends' who cuddled together, but recently Rigby has been terrorizing Charlie. He doesn't attack Charlie, but will hiss at him. At first, Charlie wouldn't come down from the cabinets (for about 2 weeks), then it was the bedroom (a month and a half), and now he is hiding under the cabinets in the bathroom. I have two separate litter boxes/food trays, but Rigby uses whichever one he sees fit & I am having to put Charlie's food/litter box in the bathroom. Charlie shakes nervously & growls at Rigby when I bring him out of the bathroom. This is getting ridiculous...and breaking my heart! Is there anything that I can do to fix this problem? I love them both very much & do not want to get rid of either of them. Is there a way to stop Rigby from being so dominant? How can I get Charlie to come out of the bathroom? I took them both to the veterinarian and was told that they are both perfectly healthy and that nothing could be done. There has to be some kind of solution...medicine..or something. They are both neutered, so that shouldn't be an issue. Thank you in advance for your advice!