Don't know how open your father might be to alternative approaches, but he might consider visiting someone who practices NAET.
NAET is an "allergy clearing" technique. It seems to hit the reset button on allergies.
NAET for me has been very helpful with my food intolerances and other allergies/sensitivities, but beause my food sensitivities are so prolific and it's not treating the underlying cause (eventually I would reacquire the cleared foods) it became too expensive of an ongoing treatment.
But I saw many people come in for seasonal allergies and were having good success.
Here's the listing of NAET practiioners from Texas: