A 15 month is not too young to spank. It’s the perfect time to begin actually. The sooner you begin, the quicker they learn to obey. Pain must be associated with sin so they will sin no more (well, not as much any ways). Use a wooden spoon on his upper thigh when he is disobeying you and say “no” to him. Yes, it will be hard at first but know that you are teaching him self-control with is a gift you are giving to him.