Hi Marco. Welcome to BYC.
Your drake is 1 year old? Did the problem come on suddenly?
I wonder if he has something stuck in his bill or throat. Are you able to hold him and look inside his bill? (You might wrap him on an old towel to help you hold him against your body with one arm and then use your other hand to open his bill and look inside). Its easiest to look with two people: one to hold and one to look.
Or maybe he has an impacted crop. Can you feel his crop at the front low down on his chest? If it is hard, its impacted. It should be soft and squigy even if he has recently eaten