brushing toddler's teeth?

okay, my 20 month old refuses to let me brush his teeth... i brush my teeth at the same time as him so he'll see me and want to mimick what i do, but every time, he just sucks all the toothpaste off.. ive switched to a different toothpaste, and he does the same thing.. i dont want to give him one that's gross, because then he wont want to even more. anyway, after i let him "brush his own teeth", i always take the toothbrush and do it myself but he only allows me to brush his tongue, but even that only lasts for about a second. I tell him to say cheese so that I can get his teeth and he either wont or he does it and then pulls away/turns his head when i get the toothbrush in his mouth... im at a loss on what to do.. his breath has smelled soo bad recently bc of it and i dont want him to end up with cavities..