Condensation above old bricked up window in bungalow

[Leebasten]

Hi,I have a bungalow which has 2 old windows on the side that the last owner has bricked up.Ive just started decorating my sons room which has 1 of these windows on a wall in the room,I put a coat of paint round and the next day the room was dry except above the old window, I think the inside has been plasterboarded so without taking the wall out I am presuming they have put a frame round and tacked maybe with some form of insulation. The damp looks to be on the level of the lintel above the window. Would an exterior air brick help ?. Many thanks for the help.