The best thing you can do is take him to the Vet for a thorough oral exam before some infection develops. These teeth may certainly be sensitive or painful and he may need a good cleaning and possible extractions. Bad teeth and poor gum health can affect the whole body and the dog's nutrition.
Teeth work will be expensive as it usually (always for extractions) required a sedative - which in the USA hads at least a couple hundred to the cost. But, if the teeth are taken care of and there is no pain, you may be able to get you dog to cooperate.
There are any, not sure how good, chewy dog products that may help keep the teeth clean. I think dry Kibble helps some, but support the idea that wet (canned) food should also be fed to dogs.
Our Westie gets a cleaning and has had one extraction in the two years since we adopted him two+ years ago.
Please be very careful of Dentastix.
I gave them to my Jack Russell years ago with no problems at all. He chewed on them fine.
I gave one to my last dog a few years ago, and she tried to swallow a huge chunk of the 'stick' and ended up choking. It was a heart-stopping moment. She recovered and was okay afterwards. But I vowed never to use Dentastix again. They are of a consistency which is very sticky and I think they are a choking hazard for some dogs.
There's also a product called Tropiclean Fresh Breath, which I'm trying on a 30 day trial right now. The idea is to pop a small amount of the gel on your finger and rub it gently onto the teeth and gums of your dog, once a day. The claim is that both teeth and gums improve substantially over a 30 day period, after which time it can be used a couple of times a week for maintenance. I am told the gel gradually softens any plaque, allowing it to be removed from the teeth from chewing and salivating.
I can't say it works yet, but hopefully it will. One of my dogs won't let me brush his teeth, so this could be an answer to the problem.
As it sounds like your dog's tooth may need more urgent attention, this may not be suitable right now. But maybe after any immediate work is done, this could help.