It would give his mouth a good clean out in teh morning - I don't really know how much is enough, but anythign is better than nothing.
The other thing to consider is sugar in his diet. If he doesn't have any/much, then again, it's less of a concern (I don't want to say it doesn't matter if he doesn't brush, but maybe it's less critical if he's not having lots of sugar)

I did buy DS a wiggles toothbrush. It worked quite well for a whiel because "wiggles brush their teeth every day"