The person who sold you the turtle doesn't know what hes talking about. If the temperature is warm enough, your turtle cannot be in hibernation. In my opinion, hes just making up crap in order to keep you calm
I'm sorry its so tough finding turtle resources in Greece

We will try to help you as best as we can
All you can really do is provide Bubbles the most comfortable environment until he gets better. This includes clean water, a basking dock and warm temperatures. I'm sorry thats all you can do for now

. Keep offering him food, but fish them out before they decay in there
Bubbles is probably not moving because of his health condition. As long as hes alive and breathing... just try your best. Hopefully, though it may take several months, keeping him in optimal conditions may improve his health. Bubbles is very grateful for you I'm sure!