My 19 month old daughter loves the fruity baby toothpaste (CVS brand, I think) and we let her watch us brushing our teeth and then she wants to do it, too. I also let her brush 1st, then say, "OK, Mommy's turn", and I get them brushed more thoroughly. Another trick we use is to sit her on our lap while she is watching a favorite TV show (probably not the best strategy, but it works). This is when I have the easiest time brushing her teeth or clipping her nails. Good luck! (Also, if you think your son has some oral-sensory issues, that could be another reason he is resistant to opening his mouth. In that case, there are some other strategies you could try, like a mouth "warm-up" where you tap or brush the outside of his mouth before asking him to open his mouth.)