25C is on the cool side but ok. Your thermometer could easily be off a degree or two so he may really be at 26C. "He's kept at a minimum 22°c because we have 40°c daily temp, I don't want to boil him! Will try adjusting it, he is happiest at 25°c." - dont understand this. Is his tank temp swinging like that? This alone could cause his problems.. shock. 22C is way to cold. The 25C is minimum. 27C should be top. You should have one constant temp whatever you choose.
You said you stopped feeding him when you hospitalized him.. so if you haven't fed him he may have nothing to poop.. How long has he been without food? Does he look bloated at all? He doesn't look bloated at all from that angle. If he's not bloated then I'd give him a couple pellets and then see if he'll poop.. Has he lost weight at all?
So not true about epsoms.. they are very safe at proper amounts and I've personally used them for over a month. Some people use them as fertilizer for their planted tanks and to remineralize ro water. It's magnesium sulfate. How much are you using?
What are the ingredients in your betta's food? Type them here please. I can't find a thing online about it.