Evening Brenda - I can sympathise - we have a wall in our study that has the same problem - its behind our desk and last summer Mr. Den did the same as your son, we stripped and painted the wall with an anti mould paint. Low and behold the same problem occurred again this winter. My neighbour has the same problem, in the same area in the same room of her bungalow.
Not sure what the answer is, but if you find one, please let me know.
By the way, how is your arm - is it getting better ?