Candida (the fungus that causes thrush), along with about 40 other different fungi, can cause fungal sinusitis, both infectious and (less frequently in persons with a normal immune system) allergic fungal sinusitis. So the answer to your question is yes, Candida, can cause sinus disease but such sinusitis is seldom associated with the oral thrush that commonly occurs with the use of oral steroids (taken by mouth or oral inhalation).
I don’t know what might be the nature of the “cluster of small white patches” you describe but do know that accurate interpretation of sinus CT’s or endoscopic findings requires a specialist’s expertise and your best bet would be to ask the ENT what he/she saw, and what you think you might have seen.
Candida in the sinuses would be treated with one or more systemic (swallowed or IV, rather than inhaled) anti-fungal drugs and, especially if the suspected diagnosis is Allergic Fungal Sinusitis, treated also (paradoxically as it may seem) with systemic steroids.
Something that is worth noting is that it is not uncommon for “sinus headaches/sinus pressure” to be erroneously attributed to sinusitis, when the cause is really migraine (or so-called “cluster”) headache. Many of these individuals, when treated with one of a class of anti-migraine drugs called “triptans” (e.g. Sumatriptan), experience relief of their “sinus headaches”).
You and your husband may want to discuss this with his primary care physician or with the ENT physician.