My Betty Boop gets her teeth brushed about 3 times a week. She has never been too resistant to the idea of getting them brushed. She would rather not, though!
She has perfectly white teeth with no plaque....yet. I've also read somewhere that the most important place to brush is between their gum and teeth where more bacteria gets trapped, although I brush both sides.
Why not try giving your baby a treat when he opens his mouth and you put the brush inside. Only give it to him when he complies. Take baby steps with him and be consistent. I've had success with training mine to do ALMOST whatever I want her to do because she knows she will get a Cheerio afterwards. If yours likes Cheerios, he'll probably be somewhat compliant, once he learns what you want him to do.
I use the very small toothbrush that pet stores sell with their toothpaste (in a package). There is a large brush on one end and a small one on the other end. I suppose a soft baby toothbrush would work. I don't see why not.
There are a couple of different flavors of pet toothpaste. Mine likes the poultry the best. She sort of likes the peanut flavor, too. Just don't use human toothpaste!!
After I brush her teeth, she always wants to drink of water. Guess they have to rinse, too.