We've been having a lot of trouble with this too. DH has found something that works though. Every evening when they have a bath together, DH will take his toothbrush and Jacob's into the bath. Whilst DH attempts to clean Jacob's teeth, he gives Jacob his toothbrush and Jacob then tries to clean DH's teeth!! I guess he enjoys that or is more responsive to it because he can see the effects of what he is doing to DH's teeth, therefore having the brush in his mouth doesn't seem quite so daunting IYKWIM???