It does seem to be near the oil gland (a.k.a. preen gland) - I cannot quite make it out . . . so it may just be the preen gland, or it could be an irritated or infected gland, or possibly something else...sometimes there are ingrown feathers, or feather cysts, I think they are called.
Has the little one been outside, where it could have gotten stung? Does the duckling quickly whip the head around to peck at or preen around the site?
Yes they're kept outside as of 2 days ago in the shed I let them out but keep an eye on them when they are but I haven't really noticed to be honest if he whips his head around. I have 7 of the little ones and they just seem to all bunch up and cutch down when I'm there. Will give you an update tomorrow if I notice anything and have spoken to the guy in the animal feed shop
I would rub a tiny tiny dab of olive oil on the gland, or at least put a warm (not hot) compress on it for a few minutes while gently holding the little in your lap. Watch out - they do jump! So keep a safe grip on the wee one.