Advice on discipline?

Hi guys, so, I have an 18 month old son, and he is CONSTANTLY bullying other kids his age. I feel as if I have tried everything and nothing is working. I’m so tired of having parents come to me and tell me I need to spank my kid because he’s a bully but I feel like that’s all I do is discipline him. I just want to know if anyone has any ideas on how to get him to stop being so mean to other kids his age. Thanks.