Yes very similar happened to me, an infection spread from a dental implant & root-canal treated molar through my upper palate & skull. Both of these perforated into my maxillary sinus cavity.
At first my fistula was a very small hole and I could cause air flow by either softly blowing into my closed mouth or softly blowing into my closed nose. Over time the hole got bigger until I removed the source of the infection, then it got smaller & closed.
For months mucus drained into my mouth adjacent the implant & molar. I could blow mucus bubbles. When I wasn't sick in bed due to the infections and could make it to work it was very dreadful and interfered with my work & school having to go spit running mucous out constantly.
An oral surgeon I consulted with called this an 'Oroantral Communication' or a 'oroantral fistula', search online for these terms.
I read somewhere that given the source of the infection is removed and it is a small hole, it should close up on its own (the body will heal it). If it is a larger hole, then surgical assistance is probably needed.
Learn how to do a saline sinus rinse, wikihow has decent instructions for it. You don't need any fancy equipment, just a cup/bowl, table salt, and water. I found the pain went away within seconds of a saline sinus rinse, and if the pain came back later in the day I'd do another rinse. For a while I did them morning and night when brushing my teeth.
I consulted with two oral surgeons. I think one referred me to the other one. He slid down a flap of gums from the cheek over the hole, then sutured it, and repeated that procedure twice more as the gum tissue died. He told me he could only do that 3 times as there is only so much cheek tissue, if he had to do it more then he'd have to take it from the palate (which I found horrifying!).
Later I had the root-canal treated molar removed (I guessed on my own that the source of the infection was in one of the roots), and then I got better. Antibiotics and the saline sinus rinses were crucial to my recovery, as was me figuring out what the source of infection was & telling a dentist to remove it; the doctors largely didn't help otherwise, they'd never seen anything like it before. The cheek flap surgeries were largely unnecessary. I had to figure everything out on my own. Without over 3 years of constant antibiotics & saline sinus rinses I'm certain I would have succumbed to the chronic infection.
Osteomyelitis? Maybe they were looking for different signs or a bigger infection? I heard popping noises, like a slow bubbling of pus within my bone. I could feel the vibration through my nearby skullbone of the 'snaps' and 'pops' of bubbling in the infected area. The ENT doctor attributed the bubbling and popping to fluids moving through the left maxillary sinus ostium... but that made no sense to me... from my senses it was clearly in my bone adjacent the implant. He was not very helpful... and he was very expensive. Maybe he did a lot of work/reading behind the scenes that I don't know about... but he was very expensive & didn't know how to help either.
I believe you when you say it ate some of his septum tissue, as it ate some of my hard palate tissue & bone around the teeth. I think I snore less due to the tissue changes, my ex-fiance said I didn't snore at all but years before my college roommate said I snored.
I think I may have managed with just saline sinus rinses actually... but no doctors told me about saline sinus rinses until years in to the trouble, they all prescribed antibiotics first. All those oral antibiotics did a number on my ability to digest things, for years after most foods I could not digest anymore, it went straight through. Have him ask his doctor if an antibiotic nasal spray would be appropriate for him.
Good luck to you and your husband & god bless you. My ex-fiance left me during this very hard time... it made my life much worse.