I agree with everyone that it's probably an aggression issue with another drake. My two runner drakes have lived together for a few years with no real issues, despite small hormonal dust-ups over the girls.
Suddenly, this spring, the senior boy, Sheldon, regularly got really mean with Layne, who ended up with an injured wing that developed an infection in the bone (and a pricey vet bill).
Since then, I have reworked the duck living quarters. The main shelter is separated from a converted dog house where Layne and "his" hen, Blair, sleep at night, separated from the rest of the flock. During the day, I alternate letting those two out of the fenced area to "free range" while Sheldon and the other girls are penned. The next day, the flock runs free, and Layne and Blair remain penned.
The boys are always separated by wire fencing. Some days they will lay side by side, with the fence between them. Other days, they will STILL attempt to fight each other through the wire. Every so often, I try letting everyone out at the same time. It has not worked well so far.
I hope you can get things settled for your boy!