I don’t have personal experience with getting kids to wear eyeglasses, and I’m not familiar with the “most common methods” of getting them to wear glasses, but I recently had some trouble getting my 7 year old (“normal” development) to wear orthotics in his shoes, so I will share what worked for us. My son is an “idiopathic” toe walker, and prior to starting therapy for the physiological reasons/consequences of walking on his toes for 4 years, he had to “break the habit” by wearing carbon inserts in his shoes.
My son was not...enthusiastic about it, since they are tricky to walk in, even for people who walk normally because the foot cannot flex. And then, it was painful because muscles were tight/overstretched or weak/underdeveloped. And finally, the flat-footed gait made him slow and clumsy, and he fell a lot. He had some legitimate reasons for noncompliance.
The solution was to limit wearing the inserts to a few hours a day at first, and to DOUBLE KNOT his “special” shoes so he couldn’t take them off unless his time was up. (We bought a brand new pair of lace up shoes that he picked out and that could accommodate the insert to try and make them more desirable.) His reward was a nice leg massage and...some iPad time. He eventually worked up to wearing them all day, every day, and he only required a massage to ease some tired legs at night (we didn’t have to keep giving him the iPad once it was a habit to walk flat footed in his “special shoes”). As an added bonus he learned to tie his shoes finally. It took a month, btw, to gain the discipline and endurance for whole days with the inserts.
I think that despite the language comprehension issues, you could try the same approach: prevent him from taking the glasses off with those sport straps (the ones that go around the head to securely attach them to the face), trying to make the glasses “cool” or personalized in some way, and limiting the amount of time wearing them to ease the initial discomfort, coupled with a reward at the end of “glasses time.” The goal of course is to work up to all day. I think that as long as you are making reasonable progress, you can rest assured you are preventing blindness from setting in.
When I started wearing glasses they gave me headaches, and it irritated my ears and nose. I have sensory issues (even as an adult!) so I can sympathize with his discomfort which is compounded by the frustration of not being able to communicate it. How does he “tell” you that he doesn’t like a certain food or that a shirt is uncomfortable? Pay attention to cues so he feels “heard” and the glasses don’t become associated with discomfort and being ignored. Are you sure they fit him properly and are comfortable? It’s worth exploring...
These are just my thoughts and experiences, without knowing what you’ve already tried and what hasn’t worked, specifically. If my answer is of no use to you, at the very least, please accept my sincere hope that you DO find the solution either here or on your own through trial and error.