My beta has one eye that is bulging. It looks kind of like this fish:
http://www.bettatalk.com/images/POP-EYE_drop_shadow.jpg

I've done some research and it was called popeye but I don't know if that's really it. I know it's just a fish and it's a toilet pet but I feel badly for him. That can't be comfortable.

He's still eating but he's shimmying too.

How can I fix it?

I don't really consider fish toilet pets... I actually feel bad for most of them.
If the eye of your fish looks like that, then it is popeye.
It could be a symptom of disease but since you said he's eating, it could be that he just needs his water changed more frequently.
Is he in a Beta bowl?
Sometimes Betas will get eye infections from dirty water, even if the water doesn't look dirty, if it's more than a day old, it's dirty.
You can try giving him fresh water everyday and see if it improves.
If it does, give fresh water about 4 times a week if not everyday.
If his water is always kept clean and you know that it's not just an infection, you can treat with medication for popeye.
Ask someone at your petshop if they sell medication for beta bowls.
Some places do and it's much easier to use rather than trying to measure down other medications made for aquariums.
If you have an aquarium, I'd suggest treating with popeye medication.
Popeye in an aquarium is almost always due to disease.
Good luck.