my boy was once like that too.
then i either get some1 he is afraid of (his daddy) to do it for him so that he guai guai brush or i tell him "if you dont wanna brush by me then i get daddy to brush" then he scared and obeyed, or i bring his favourite toy and brush his toy's mouth, then he wants it also! Sometimes has to do some acting and silly tactics...coz at this age they believe everything you say and do. Or another resort is you buy the toothpaste with the cartoon character that they liked...(winnie the pooh or disney pixar cars by colgate etc) so that it helps to make them look 4ward to brushing teeth.
For mine was quite difficult to work at first, then i just use the hard way - better brush or i scold....kekeke...or i brushed for him chop chop, even tho he refuses or struggles, hahahazzz....or some days really cannot fight with him then use a towel to wipe away the yellowness etc...of coz with him struggling/pushing/kicking also...
now he is okay already, and he loves to brush his teeth and rinse his mouth. Somehow he thinks its very fun to rinse coz he can play with water....like a fountain.