I have 2 autistic boys who were using dummies for longer than normal 6 and 7 to be precise, not good at all but I can understand your difficulties.
What I used to do was buy new ones every so often, then to wean them off I refused to buy them new ones so they had to keep the dummy until it was really bad, gradually this weaned them off dummies (because they were not nice anymore)
eventually both boys stopped using them so much and got used to being without them. try it, my sons are largely non verbal, and have the understanding of a toddler. but for their teeths future, its worth trying..