HELP!!!Erratic Behavior from our Delta Tail Betta fish!

[Marianne Betta Fish Owner]

Hi! My son's Delta Tail Betta fish is not feeling well and are acting very strange. We have only had him for about 3 weeks but we have read a lot about the care, cleaning etc and the fish is taken very good care of....We hope, but he has not been interested in food for a long time (2 weeks +) and first we were told that we should try some different food types. We have tried Aqueon Betta Food, BettaMin and freeze dried Bloodworm (only one at the time as a snack-No Interest!). Delta has been staying/resting a lot under the filter and not been very active swimming...less and less. We have been to the fish shop and asked a few times already and where told that Betta fish does rest a lot, some almost all the time. I think that sounds strange as fish normally is very curious... I have now noticed that he is ''resting on the side, sometimes it is like he is resting on his back, he seems to breath slowly and when we changed his water earlier today he was swimming very strange, almost like he did not have any direction control and his body was tilted to one side. We are so worried for him and don't want him to be sick, suffer or die so Please, can anyone help us with a diagnose and what we can do to help him?It looks like he is twisting/bending his body to the right. When he is suddenly swimming out (very seldom) it is almost as if he is ''crazy''. Is he in pain...what can this be? The water and everything inside the tank is clean, pH is < 7.2 and Ammonia 0 ppm. No food rests. Hope someone can help Delta and us with a good treatment! Kind regards, Marianne (Delta's ''Grandmother'')