I'm looking for a scenario where a father can potentially be the Kohen for his own son's Pidyon Haben.
Normally, this would be impossible, because a Kohen is pattur from Pidyon Haben. Is there any scenario where this would be possible?
If the son is a challal. If a Kohen fathers a challal, the son is not a Kohen and therefore needs to be redeemed. However, there is no 'ceremony', because the father is זוכה בפדיונו automatically. So he is the Kohen, but there is no actual Pidyon. (See Bechoros 47b)