He said he had already asked SS and he said of course he was brushing.
 

These kids be lying 

Not saying that to be mean but they all do it to get adults off their backs about chores, hygiene, etc

Yes I'd stand there and lay eyes on them doing it until they gets tired of being micromanaged. 
 

I had to do this with my bio about washing her face  she had horrible acne and tried lying about washing her face so I started making her show me the proof she'd wash her face till she got tired of being micromanaged 

As far as teeth are concerned 

I told my bio "look here little Lilly jr ... you can lie all you want to but those dentists and those X-rays will have the truth and if they catch you with cavities then YOU pay for those filings with your money" 

Hit them where it hurts ... in their chore money / allowance wallets!!!

Once their behavior starts affecting them/their pockets ... oh best believe they will turn it around 

***if positive reinforcement hasn't worked then unfortunately gotta resort to age appropriate consequences/negative reinforcement to change the behavior 

My formerSO's kid wouldn't brush his teeth either. Also, he wouldn't use toilet paper. So he'd poo and then take a bath. Marinating in in poo water. Of course then he'd go into the kitchen and use his fingers to get some peanut butter. 

Bleh. So glad I left that behind and never looked back. I couldn't trust any food that was open. 

Nothing is more influential to a teenager than fear of public opinion. "Well SS, no girl is going to want to date you if you have yellow, plaque filled teeth and nasty breath." It would be even more effective if a peer called him out on nasty breath. He would never not brush again. 

2 to 3 years and you will have no say in SS life,   It's DH the BF job to parent his kid.  He doesn't care.  You can't do more then the BP.  Accept the fact that SS is lying.  And it's his teeth. In 3 years 

My skids are gross too. SD10 especially. They go days without any shower or brushing of teeth. Then the night before they go back to BM's SOMETIMES SD will want a bath. This annoys me because after she drains the water.. on the bottom of the bath has a film of her 4 days of no shower filth. It's gross. I scrub it clean afterwards of course, but it's annoying. 
Oh and she insists on growing out her hair long, but doesn't brush or wash it. When she does wash it she uses soooo much conditioner that it leaves clumps on my tiles, refuses to use a hair dryer and goes to bed with dripping wet hair, soaking my linen with her wet head, and the next day wears it out and unbrushed with her ends all tattered. Like wtf is the point of that?! Anyway, she thinks I'm a snob because I have standards in hygiene. BM is a pig aswell so it's a lost battle and I now just use her unhygienic laziness habits as an example of NOT what to do for my 3 year old. 

When Spawn was living with us we took her to the dentist regularly and made sure we took her to the doctors for updates on vaccines and if she was sick.  She moved out to live with her maternal grandmother when she was 16 ½ and starting her senior year.  Towards the end of her senior year I got a phone call from the dentist to confirm her appt for several cavity fills and a root canal.  I was stunned, she must not have been brushing her teeth at all.  The last check up she had was over the summer when she was still living with us and had no issues.  I know she started smoking pot and vaping and doing who knows what else once she moved out, but it was just shocking to see how quickly that happened.   These kids think they are Teflon and don’t think about how their lack of actions have consequences. 

Stop tolerating the lies. When you or SO ask about SS brushing his teeth, hand him a disclosing tablet to chew up. It will die any paque bright florescent Red. Then call him  the liars that he is and send him to brush and floss until there is not a single trace of dye.  It will take potentially hours to get all of the dye off of their teeth because every molecule of plaque has to be removed to get rid of the dye.

Disclosing Tablets _ There are several brands available. Here is an example.

https://www.net32.com/ec/trace-disclosing-tablets-250-d-45427?vid=20811&...

We had the hygiene issue with SS though he was manic about showering and brushing.  For some reason he stunk like wet stinky dog any time he "supposedly" showered. 

We would send him back to shower again. Sometimes several times to no avail. Wet dog was his odor.

Finally he came downstairs for dinner one evening. He was in his late teens.  He had just showred and just reeked to high heaven.

I told him to get his butt upstairs, put on his bathing suit, and meet me in his bathroom.  I had him shawer under my hairy eyeball.

First he washed his hair, then his body.  Just like he had been trained to do a decade + before.  It was all I could do not to bust a gut laughing my ass off as he showered.  

He still reeked like wet dog.  Why one may ask?  He would take his shampoo bottle, hold it over his head, turn it upside down squeeze, then lather and rinse.  Then he would grab his body wash bottle, put it in a scrub puff, lather up and wash his body.

He got so mad when I called him wet dog man again.  After some irritation on his part I told him to grab his shampoo bottle, squeeze some in his hand and show it to me. Surprise. The bottle of shampoo was empty.  The look on his face was classic and I did a full belly ROFLMAO at the look on his face when he realized he had not actually shampood his hair in who knows how long. 

When he needed body wash, shampoo, toothpaste he would write it on the kitchen shopping list sheet on the refrigerator.  DW had not bought him shampoo in ages because he never wrote it on the list.

I reminded him that what he had just experienced is why he was shown to put the shampoo in his hand then lather it into his hair.  He never ran out of body wash because he saw it lather up when he washed.  When the bottle was getting low, he knew it and would put it on the list. Same with toothpaste.

His mom and I still will occassionally give him shit about the squeeze the empty shampoo bottle on his head learning event.

Never again has his head smelled like wet dog.

He is exceptionally cleanliness and appearance conscious.