I totally agree with Amelia. For one thing, cavities are not always preventable. I adopted a kid who lived in an orhanage and never brushed his teeth in his life until coming here and I had a biological child who brushed. I was really afraid that the adopted child's dental appointment would be awful. It was...lol. My biological son, who brushed, had six cavities (my family has notoriously bad teeth. I had...ready?...TWENTY THREE cavities the first time I went to the dentist. Now my mom let me drink a bottle with chocolate milk in it, sot hat's why so many, but some kids just don't get cavities). Which brings us to the adopted child who never brushed. He had no cavities.To this day, he has no cavities.
I personally don't think this is worth any punishment at all, especially with this child's disabilities. He can not be expected to see life the way you do and I'd relax the standards a bit. You can't make him not have disabilities by punishing him. You need to get him the proper supports and let some things go. My 21 year old son is on the autism spectrum and doing well. It took him almost until twenty to see the wisdom in hygiene. Autism is a developmental delay, even if the child is intellectually bright or average.
I agree that teens, even the best teens, are not always going to jump when you say they should jump. It's normal. You will feel better if you relax some of your standards. So I agree with your husband. Oh, yeah. I also think writing sentences is non-productive and a unhelpful punishment. Has it helped yet?..Are any of you in therapy?
Good luck 
