What I would do**
is put him in a one or 2 gallon container ("hospital tank") and add some aquarium salt. Instructions on the salt container say half teaspon per gallon. It's like giving a sick child pedialyte
If he looks a little better after a day or 2 put him back in the 10. If not, do a water change.....then you might need to try some other fish meds. No one likes the one I used so I hope someone will come along and suggest a good one.
I've only treated one sick betta in my betta career so far. I did a good job because he revived in 2 days and is super healthy now.