Thanks for everyone’s suggestions and reassurance, we’re just going to keep a close eye on him and if he begins to act different or show signs of infection we’ll take him straight to a vet!
We think we’ve found the cause though; this boy and another (that sadly didn’t make it) were abandoned by a river as tiny ducklings and we saved them. Now as Lucky has turned out to be a boy we theorise that someone vent sexed them without knowing the proper way of doing it and so damaged them. This would explain why they were dumped and why lucky has this problem now! Such a shame nasty people exist
Still, he seems to be in good spirits and is as playful as ever so he’s mostly unfazed by this issue!