While he may also have popeye it definitely sounds like he has SBD issues on top of it.
Epsom salt is the best cure for now. I have a fish that I rescued with SBD and she exhibited the exact symptoms your little guy is. I had to bump her to 2 tsp/gal of Epsom salt. I would recommend bumping him up to that dosage for a while. It may take a few days. My fish refused to eat for four days after I brought her home, and I have no idea how long she'd gone before that. He'll eat once he feels better :)
I believe the salts should help the swelling with his eye as well.
- Warm water: warmer is better, 80 degrees is good for healing - if you can get a heater it will really speed up his healing process. I liked the Tetra ones (not adjustable, but good for now. Just make sure you have a thermometer, too!)
- Epsom salt: dose is up to you depending on his condition - no more than 3 tsp/gal though. Remember it will take a few days to have an affect. At this point I recommend 2 tsp/gal
- Lots of rest: another reason to get a heater - this way you can keep his light off longer so he can rest
- Frequent water changes: with salt it's best to do daily changes anyway
- I highly recommend using Stress Coat+ in the water. It helps keep him calm and will help with healing and slime coat production. Also if you happen to have indian almond leaves they help, but they're not a requirement :)
- Provide places near the surface for him to rest. Plants, betta hammock, etc. so that he isn't struggling to the surface for air
- If he doesn't eat right away don't freak out. He should eat when he's ready. The best way to get him eating again is with live/frozen food that's particularly delicious looking. Bloodworms are good, as well as daphnia. Daphnia will act as a natural laxative to help him pass whatever's blocking him up (a better alternative to the pea, which bettas cannot process)
My fish became very hyperactive the better she got. She would dart around and try to swim down (she was a floater, not a sinker). It's possible this may be normal, but keep an eye on him anyway.
If something comes up let me know. I'll help if I can, or find somebody else who can! Good luck with your fish. SBD is usually treatable.