Any fish experts????

I was looking for a Betta fish today and I came across a little dragon scale Betta who was swimming on his side. When I picked up his cup he would swim normal then go back to his side. I asked the manager about him and she said a little boy had help his cup and dropped him on the ground and he had been like that for 5 days already. she said he was eating and pooping.

I felt real bad and I decided to try to rescue him I put him in my 5 gal heated and filtered. He doesn’t look much better. Any thought? Is he dying should I end his misery? All he does is swim on his side.