He looks completely fine on that video. And swimming fine, it just his tail is heavy that why it looks like he swims and twitch. And his color is perfectly fine on that vidio. I bought a lot of bettas for myself and other people , about 30. And i did have problem with about 6 of them. As soon as you give them blood worms they will refused pellets or any other food. One time it literally took me 3 or even longer weeks. But it easier for me because i have them on my table and i literally offered food 25 times a day. So if he is eating blood worms and swimming the way on that video he is fine. Just keep offering and remove uneaten food so it don't contaminate the water. You can try to offer pellet , you can try to chip them to tiny pieces,crush it. Fast him a few days.
You saying that he is stressed. How big is his hospital tank? How often you do water changes. If you will acclimate him right way he will not be stressed. If it hospital tank you need to do 50% and 100% water changes a week. When you do 100% water changes you need to make sure new water match the old water temperature so you don't shock him with drastic temperature difference. Do you acclimate him if you do 100% water changes? Sorry i read briefly all threads if he is in small container then you need daily water changes.
For you tank you really recommended to do 50% a week with gravel vacuuming and filter media rinse/swish in the old water WITH a water change.