My son, 8, just won't let me do it. He has ASD and other problems.
He will waft a toothbrush round generally but not let me near him. We saw a doctor yesterday who had a quick look in his mouth (literally seconds before he screamed) and he is already showing signs of gum disease. 
I've tried different brushes (including electric), reward charts, different toothpastes.
He swallows any teeth that come out - not sure that is relevant but I thought I would throw it in.
Any ideas? Or is this just something I have to put up with. I can't pin him down. He won't open his mouth and it seems like cruelty.