He look fine to me too. I really think its the part of his coloration. If he would have a velvet he will develop more symptoms like clamped fins, he might dart around , he might scratch the body on objects. So watch for the symptoms. But so far he looks fine. Never treat fish with anything untill you know he has it. Salt is good for the treatment but still you don't want to stress him out with any treatment. Btw he was leaving 11 month in uncycled tank so i would recommend to keep his 2.5 gall tank uncycled without filter. Do not change anything in his care really. He is happy for 11 month continue do what you was doing. Almost all my tanks are 2.5 gall without filters and my fish live long. Do 2-50 and 1-100% water changes a week or you can do what i do one full 100% water change every 5 days.
The food is very important. Some food has a lot of feelings and not good for them. What food are you using?
Make sure you always acclimate your betta slowly to the new water so you don't stress him out.