Hi and thanks for the query.
There is a wide range of causes that comes to mind when Phantosmia (perception of smells that are not there) occurs.
The causes are central as well as peripheral.
They can be due to the blocked sinuses and nasal congestion from his hay fever. This can cause altered sense of smell and increasingly possible in someone who has hyperosmia (heightened sense of smell). Central causes such as seizures or temporal lobe problems, tumor of the olfactory nerve, transient ischemic attacks (mini strokes) etc. are all responsible.
He needs a comprehensive examination and a detailed evaluation - referral to the ENT doctor will be the best course of action.
Let me know if you have any queries.
Please do leave a positive rating for the service by clicking on the stars on top of your screen.