I have pollen allergies and if the antihistamines don't work, my advice is to tell him to stand and endure as best he can. Wear a face mask to block the pollen particles, wash his clothes often to keep them clean of pollen and avoid people and places that are laden with pollen. Medicines and supplements, even when they work, stop working after a time. Did you know that many supplements are contaminated with pollen? If he swallow pollen spiked pills, they could rip up his insides if the allergy is severe enough.
I am not a Christian, but many Christians believe that prayer is all you need to treat any illness, that doctors and medicines are poor substitutes for the power of the Supreme Being. If you are Christian, they would advise you to forget the liver cleanse and say prayers over and over and over for all your illnesses. If he can't endure the allergy and he doesn't want to try any other treatments, he can pray.