Well, we tried some things today. First of all we bathed him and he did his poop here, it was normal, healthy looking poop, so not worries here. I know for sure that he ate one roach and one cricket as after 2-3 days they were gone, and I also guess he ate some small mealworms that I tossed into his tub. He's also rounder than before, so I know he eats at least a little, even if I don't see it. My main concern is the lack of calcium as like I said he eats his insects after such a time that they rub off all the calcium in the meantime.
Today while bathing him I cleaned his tub, switched it back to clean-hospital style, and then we tried the separate container method. I leaved him in it for 2 hours with one cricket in a dark room, but he refused to eat it. We decided to try to give him pinky like his breeder told us when we bought him (rub the pinky on the lower lip) and even some teasing on tongs (like "look, I'm just a live pinky walking there"), but again he refused. We'll leave him alone for few days now so he will be able to calm down as I guess he was pretty stressed today after all that. In Friday we'll go for our reptile shopping and just buy him one live pinky to see if he'll react to the movement. I'm normally against feeding live but after breeding snakes I know that it's often the best way to get them to recognize the shape of pinky as a food.
Edit: Pics I took few minutes ago, to my eyes he looks quite healthy and certainly got more round than before.
Also, I'll try to feed him some liquid calcium tomorrow. Is there a possibility that I may use too much of it? I'm not really sure how much calcium to water ratio I should use, and how much to give him? Thanks for all the tips, I never imagined that owning a pacman will be so stressing to both me and him!