You've pretty much done all the right things, and like your husband, I once had a condition that resisted everything. Out of frustration, I developed the flip-turn sinus flush. If your husband has tried a saline irrigation, it's not going to do anything for a frontal infection, because gravity prevents the saline from reaching that high up in the sinuses. The flip-turn actually uses gravity to get saline there.
The flush works best for bacterial infections, but after I finally got rid of it, I slept in a moldy cabin on a 5th grade camp week, and got a mold infection. It was actually more difficult to flush out, because the mucus was clear and wasn't giving much feedback the way yellow mucus does. Of course, there's about 40 different fungi or mold that can grow there, so there's not much of a guarantee I can give you.
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground. If your husband is over 55 or is worried about falling, there's a reverse flush (chapter 13) that carries less risk.
The flip-turn kind of ties into research done by the Mayo Clinic. The same team is also connected to the discovery of the widespread presence of fungus or mold in sinus infections, and may possibly be involved in the trial anti-mold drug. Please let us know how well that works.
Good luck