Welcome to the Forum..
Your English is vey good. Your Mr. Darcy is just adorable. I am sorry that he has been so sick for so long.
Do you know what kind of shots your Vet has been giving him? Is it Corticosteroids?
Did the Vet tell you in detail what foods to cook for him and what foods to avoid?
I am wondering if Darcy has chronic Inflammatory Bowel Syndrome. This can cause pain and diarrhea along with malabsorption [innability to absorb nutrients from food] which can lead to skin and hair problems.
On the other hand, Mr Darcy could have severe food allergies. Besdies pain and diarrhea, does he have any other symptoms? Is he very itchy and scratching all the time? That might explain the hair all over your flat. No, it is not normal for Havanese to shed like that.
Have you talked to Mr. Darcy's breeder [I am assuming that you got him directly from a breeder.] to find out if any of his relatives are having similar problems? A responsible breeder would want to know what is going on with him.
Please let us know what the new Vet says about mr. Darcy's condition.
Best of luck to you both.