Short update:
I decided to wait until the end of this week. If he didn't get any better, I would've taken him to the vet BUT this night he left his cold hide again and crawled around. This morning I found him in his warm hide so I decided to check if the cold side was dirty. To my (half) surprise I actually found his shed (one big piece). I'm sorry for troubling you since I know that you guys were worried about him as well but it seems that his bad mood and absolute refusal to move and come out were due to him being in shed.