Cats not getting along

Hello. I have 2 cats. Both male neutered indoor one 8 one 6. I have had them since they were both kittens, so they have been together 6 years. They have always had their temper tantrums with eachother, nothing to serious, just cats being cats. The younger cat has always been the most dominant of the 2. About a year and a half ago I had to move back home, so the cats came with me. This was not a new place to them, since they had lived there before. About 9 months after moving back they started really fighting. I wasn't home alot (but they were never alone) and I thought this might be why. My older cat would hiss and growl at the younger one everytime he came into the room. All I can assume is that they got into a horrible fight at some point. I was finally able to move out 2 months ago. I was hoping by doing this and bringing them into a new place they might have forgotten what happened and get along again. Well this did not happen. My older cat is now horrified of my younger cat. He is starting to urinate around the house because the younger cat just won't let him near the litter boxes. The younger cat is always kicking him out of where ever he is sitting/laying, for no apparent reason. My older cat can't go anywhere in the house without being scared. I don't know what to do and I am scared that I will have to get rid of my younger cat. I DO NOT want to have to do that!! He is a great cat otherwise. Any advice? Tips? Tricks? I will try anything! Thanks so much!!!

Thanks for the advice! I will give it a try and see how it goes. My older cat has an appt coming up soon so I will be getting that done! Thanks again!!