I am not 100% sure, but also thinking might be biting, or due to the water quality before. I think that its good that you doing more water changes now.
One suggestion , since you don't have the filter which is fine, i don't have them either, but i would really recommend to do 2-50% and 1-100 % water changes a week. Or at least 1-50% and 1-full water change a week. If he is tail biter you need to keep water super clean to prevent infection.
Also good nutritional food will help . The one that you feeding him is really not a best choice at all. I would really recommend to buy those kind
Also NLS has garlic in it (omega one does not ) which is also works as a preventative for the infection.
Also NLS Thera A has a mega dose of garlic 1 mm size of the pellets would be good for your betta.
Also if you will do full water changes you need to acclimate your betta to the new water so you don't shock him with the new temp diffirence. So if you need instructions let me know. But i would really recommend to do full water changes at least once a wk if you want to keep him healthy longer.