My Jakey has been biting his tail.

He started about a month ago and it's been getting worse. I figured he was bored so I upgraded him to a 5 gal tank (from a 2 gal) but his tail is still getting worse. I haven't seen him biting it, but it's not finrot, there's nothing sharp in his tank, and I covered the filter intake but it keeps getting shorter. At first he was just losing the spikes of his crown tail, but today he took out a chunk of the solid part too. I just gave him a ton of live plants and a tunnel this morning but his tail is shorter again since I added them.
Is there anything else I can add to keep him from getting bored? Would it maybe be better to move him into the 20 gal divided so the other bettas distract him? He's on my nightstand now, though, which is where he can get the most attention since I do all of my reading and studying on my bed.
I've been treating him with Vita-chem, garlic guard, and stress coat, but since he's still chewing his tail down it doesn't get a chance to grow back. (by the way, can anyone tell me how much garlic guard I can add to the water and how often? there aren't good instructions on the bottle)
Sorry for the rambling post. I'm just so frustrated with all of this. Jakey is my little baby and it hurts me to see him hurting like this.
