By insisting on it every time he plays on an object with wheels and by modeling it yourself every time you ride.
Toddlers want to be just like us, so one of the best ways to ensure compliance with bicycle helmet use is to model the use ourselves. Make sure the helmet fits snugly but comfortably then put one on your own head. Helmets should be sized according to the age of your child and then worn square on the head with the forehead covered and chin strap secured. It shouldn’t wiggle but needs to be comfortable as well.
Look for a label indicating Consumer Product Safety Commission (CPSC) compliance on the helmet before purchase. Toddlers and preschoolers should wear helmets when on bicycles, scooters, big wheels, and the like. Make your home a “no helmet, no ride” household, and soon your child will not even think twice before putting on his helmet.