Betta Fish: Is my fish dying? :( spoilered just in case

I’m a college student, left for 3 days for thanksgiving break and came back to this. He’s not eating (he typically doesn’t eat if it gets too late so I’m not worried about that) and he’s always pretty lazy but it’s different this time, idk why he’s on his side like that, and I’m super worried. Admittedly I think it’s my fault (I have not been doing water changes as much as I should) but even then, I checked the water parameters and they were fine because I have a pothos cutting (0 ammonia, 0 nitrite, 10-15 ppm nitrate). I did a 10 min salt bath and did a 75% water change but there’s nothing else I can do at this point