always place baby on his back when you lay him down for sleep. it's ok if he turns to his side on his own. you can flip him back if you prefer but he probably just likes sleeping on his left more. you can work on side lying on his right when he is awake. when on his back, you can encourage him to turn to his right using an attractive toy.
Try calling him or giving toys and playcards to the right. Attract his attention to the right side so he turns the other side. Dont worry, they will take their time to master the other side.