Paradise, your tanks look great!! Just read through this thread
I had a rubberlip in my smaller tank, and I had problems getting him to eat sometimes too. Then I read somewhere that though they are omnivores, they lean much more to the meat side of things I guess. Try some carnivore pellets for him maybe? I'm sure the hikari sinking pellets might work too. They also need a really good amount of flow...
I'm sure he'll be fine, from my experience I think they're just tempermental. Did you move him before he started not eating, that could be it too.
Again, your tanks look great, I love the hardscape and choice of plants as well