From your description this sounds like black mold which is not deemed to be a cause of dermatitis. However, with the flooding, there could be growth of countless other molds or bacteria. However, unless your dad has direct contact with these organisms, they would be an unlikely cause. His having suggested that the problem is "some sort of allergy", the dermatologist should be asked, after taking a good history of all exposures, including at work, what might be the 5 most likely causes.
Hi, I don't know whether the black mold is causing the allergy but is funny you have posted this suggestion because I have just been diagnosed with asthma at the ripe old age of 33 and have been using a make shift office which also has black mold on the roof.
I am going to blitz it and hope for the best. Please keep me posted.