How to help my baby and dogs get along

My son is 9.5 months old and loves our two dogs. We have an almost two year old pit, named Milli, and a baby pit, named Steff. Milli usually runs when she sees my son coming. He squeals and grabs her neck fat and her ears. So I don't blame her for trying to avoid him. He does the same to Steff but Steff doesn't seem too bothered by it yet. I tell my son "No!" or "Stop!" when he pulls on them but he doesn't understand he's hurting them. How do I teach him to play nice?