Afraid your vet is right. Often these are related to seasonal allergies, and allergy testing may turn up things that can't be avoided, like grass and dust
Like with humans, allergies can develop later in life, although dogs allergies show through their skin rather than their sinus like humans.
Very hard to say if this is something he'll always have problems with. He may only have it at certain times of year, or only when certain plants or grasses are in a phase that aggravates his allergies.
Be very careful about walking him anywhere that has been chemically treated and certain kinds of garden mulch can cause allergic reactions. You may want to wash his feet after walks with hibiscrub or something similar, and dry very thoroughly.
You may also want to research homeopathic remedies. You have to match the remedy to the symptom, so you'll need a good book on veterinary homeopathy, but the remedy Rhus Tox is a very good place to start for skin allergies
