Thanks everyone, but it looks like he's going back downhill. It IS killing me to watch this, not knowing if he's going to pull through. I do have a plant that can be put in there, so I'll put it in now.
Sherbet has been my healthiest betta, so I have hope for him! He's pretty much the only betta who hasn't gotten sick in some way or another (: That PROVES that he's a strong betta... he just needs to fight, fight, fight!
And on an even sadder note, I went to Petco today to get another Critter Keeper, and was disgusted by how the bettas were. There were about 5-6 dead or dying, the water was less than an inch high, all of them had algae in their cups, and almost all of them had some kind of disease. I wanted to get one so bad, so help it out, but I couldn't, not with Sherbet sick right now. If he passes on, though, I'll make room for one more. If I can. D: He's my baby, and I don't think I could replace him...