My daughter is 19 mths and she loves brushing her teeth in the bath, she thinks she's clever cause she says 'more' dips it in the water and keeps brushing! Also, I found that she brushes better if she does it at the same time as me or my husband, she loves being the 'same as daddy'!!
I wouldn't stress over it too much though,he'll start brushing soon, maybe leave it for a day or so and he might ask for it.
Hope this helps