It is not right to say that teeth cannot be brushed before attaining six years age.
Your child can be taught to rub his teeth by his own fingers. For this you should show it first by using your fingers softly. Ask him to watch how you brush your teeth with your fingers.
Brushes with hard bristles will injure his gums and teeth. But select a very soft baby brush available in market. If it pains him avoid and let him continue to use only his own fingers.
Teach him to gargle and spit without blocking. Let him not choke, but do not mind if he swallows even the sweet toothpaste or tooth powder. In the absence of brush the tooth powder can give better abrasiveness than the toothpaste. Teach him to wash his mouth after meals properly and spit.
By and by the child will get used to the cleaning of teeth properly.
Visit qualified and experienced dentist periodically, taking advice from your regular paediatrician also.