How to take care of a young child's teeth if he won't cooperate?

Make it routine. Routines always help, as kids know what comes next. May i suggest you get 2 toothbrushes one that your child uses and the other one that you "help" him out with. That way you give him a bit of control and you teach him the basics at the same time. Brush your teeth together, so he can associate the activity as a "family" one!
Pediatric dentist. A good pediatric dentist can demonstraet it to you.