i've used plaque off and logic oral gel on my cats/dogs before they switched to raw. i've also had a vet flick the plaque off teeth using his thumbs. ive also tried to brush their teeth.
i would suggest trialling plaque off and i have seen it work with my lot, but that a dental may actually be necessary.
my eldest ended up having to have a dental, and it has really helped him. his teeth were in much worse condition than either the vets or i knew about, and by having a proper look the vet was able to determine that the best course of action for him would be total extraction except for his canines. (in fact, most of them were broken / and or had already been lost!)
i felt really really guilty for putting him through that and not realizing his teeth were so bad, but afterwards, he didnt notice he didnt have teeth, and is now an absolute pro and eating raw food. And, i consoled myself that as a rescued animal, he may not of had the best start in life, but from this point forward, he could have the best life i could give him).
if the plaque is currently only on his back teeth, it might be worth trying less invasive methods first before having him going under GA?