Don’t start with a brush, start with just putting a finger in his mouth with a bit of the paste, just let him lick it. When he’s ok with that you can try a bit of gauze around your finger and “brush” with that. Work up to the brush, always letting him sniff and lick every new thing you introduce. You may need to try several flavors to find something he likes. I use a paste I make myself with just a bit of baking soda and some coconut oil. Coconut oil is very healthy for a lot of reasons. I use a finger brush , I feel I have more control with that rather than a regular brush. Don’t worry about getting the whole tooth, the outside is more important than trying to get to the inside surface.
There are also some additives you may want to look into that are supposed to prevent the plaque from attaching to the tooth to begin with, they’re made of a seaweed extract. Not a substitute but in addition to brushing. You just sprinkle on food each day.
Last but not least, a raw bone to chew is a great toothbrush if you’re open to that. There’s a lot of good info here on feeding raw bones for dental health. My guy gets a bone at least twice a week. He’s 12 and has never had a dental.
