With Bo, (who acted exactly like yours) I used epsom salt. One tablespoon in 2 gallons of water, no tetracycline or anything, for four days. Also, keep it a little dark. I did change with water 100% every evening and I feared stressign him so here is what I did:
2-gallon water container
Small bowl heater
Large net- Bo was NEVER caught with this net, it was just to get him away from the light ASAP.
Hospital tank (3-gallon critter keeper with no filter since I'm going to do 100% daily water changes.)
1. Put water into 2-gallon holder.
2. Squish small betta bowl eater into 2-gallon holder.
3. Plug in heater, add salt and conditioner, and wait a few hours (I do this the night before, after I've changed his water out.)
4. Use your spare thermometer and run your sink water into a bowl until it gets warm-ish.
5. Put thermometer into bowl, and try to get the temperature to be similar to the water in the aquarium, and in the 2-gallo container (fewer temperature changes + properly treated water = happier fish).
6. Unplug the heater and lower betta cup (that he can see out of) into water. Don't chase him, wait for him to swim near it. Lift the cup upward and hopefully you'll get him inside it. If he won't move from the corner, use the other end of the net to gently push AT him (you probably know not to touch the fish, but ya never know who might be reading the forum).
7. If he's being a pain then get the heater out and wash it by swishing it around the bowl in the sink until you feel safe to pull it out and dry it with a paper towel
8. Try to get him in the cup again, just being sure to be patient and gentle.
9. Repeat step 8 as needed. If he just won't move then turn the cup upside down, lift it out of the water, turn it sideways and push it into the water near him. The current should sweep him out of the corner and into the cup, but make sure you try to do this as slowly as possible.
10. Float the cup in the tank and put the net around it- leaving the sky open.
11. Remove any ornaments that you want to clean and put back in the tank- give up on gravel.... trust me.
12. Start removing water until you feel you can dump out the whole tank - this is why small hospital tanks (2-3 gallons) are awesome.
13. If your fish is a jumper then net the top of the cup, otherwise you can leave it open. Mine has never jumped, but it might be best to just net the top anyway.
14. Remove net and cup (hold the cup, don't just let the net lift it) and place it into that bowl in your sink that is roughly the same temp as your tank, or place it onto a plate (my fish is in the kitchen...)
15. Dump aquarium, rinse, wipe with paper towels to remove unseen gunk. Place tank back in it's original location. This should take about a minute since you're not sanitizing, just rinsing.
16. Put water from 2-gallon into tank, with any decor you want to keep in there, the heater, and the thermometer.
Dump about 1/2 of the water out of the betta cup, and move it, in the net, to the aquarium.
17. Float cup and add some of the new water every 15 minutes.
18. Lift cup and cet, dump 1/2 of water. Repeat step 17.
19. Repeat 17 and 18.
20. Lower net and slowly tip the cup or just lower it until the fish swims out the top. Sometimes I'll teach the bottom of the tank and Bo will still be in the cup. I just walk away at this point.
Whole change takes 50minutes, but the fish is only "out-of-water" for one minute and the temperature does not vary. This is the lease stressful way I've found. But fish are very emotive. Sometimes Bo handles this without a care, other times he won't get in the cup or he hides and ... well let's just say he's full of energy now.