I second the recommendation that Darcy's humanpal look up Darcy's
symptoms in Pitcairn's book and see if they match up for a homeopathic
remedy. I know you've said that herbal allergy meds have been tried,
with little help, but perhaps he needs something for asthma or chronic
respiratory infections.
Another suggestion--keep him going as much as possible without
antibiotics, as unless he has verifiable bacterial infections,
antibiotics are no use. Viral stuff can't be cured with antibiotics,
and in general antibiotics can become fairly ineffective if used over
extended periods of time. There is speculation recently that physicians
overprescribing antibiotics has led to all sorts of "mutated" germs that
can no longer be killed with the antibiotics we have on hand. My vet is
one who will not prescribe antibiotics any more for my cats unless he
knows he can explain fully why he is doing so and convince me that they
will tackle an existing bacterial infection.
Also, have you checked at all to ensure that Darcy is not simply
allergic to something in his food? I don't have first hand experience
with food allergies and cats, but know of people whose furfriends
suffered from wheezing and etc. because of allergies to corn, turkey,
and a variety of other typical cat food ingredients. You might try
switching foods in order to see if that helps at all.
Good luck--
Gabby and los gatos fregados (the darn cats!)