speaking from experience, my pointer was not into having her teeth brushed (she was 8 when i rescues her and never had it done before) i talked to the dental tech where i worked for some help. he said wrap your finger in gauze and try that, sometimes they just don't like having an object stuffed in there. make it a positive experience (lots of goooood giiiiiiiirrrrrlllllll!!!)
wrap him up in a towel like a burrito have someone help you hold him and try the gauze (the toothpaste is optional if your dog hates it don't force it) you'll be surprised how that gauze can clean! also use the c.e.t dental chews. those are the only chew out there that i will back cause i've seen it with my own eyes how much of a difference they can be. my pointer was going to have to get a dental cleaning but i said give me a month to get her mouth into shape, all i used was gauze, my finger, lots of patience and persistence and those c.e.t chews and her teeth were excellent!they took before and after photos of her teeth to use in demonstrations that is how much it worked.