What should i do to motivate my son to brush his teeth every night?

Simple: Just show him aunt nellie who wears false teeth. Then tell him he does not need to brush all of his teeth just the ones he wants to keep.

Make it fun: Create a game which is fun... There is a cute video of a chimpanzee brushing and flossing. Take your child to a pedodontist who will help create good healthy habits & make dentist visits happy & fun as possible. If you create a habit of going a minimum of 2 times each year and show your child the same example for you and his other parent. Be very positive when you speak about going to the dentist.

Brushing: Make it a family affair. Brush with him. Play his favorite song on your smart phone and both of you brush the entire length of the song. Your son should be brushing twice a day for 2 full minutes by the clock. You should be flossing him once a day to clean where no toothbrush can reach. He MUST see Dentist (best would be Pediatric Dental Specialist) 2x/year beginning at 1st b'day.