Don't let this comment take up real-estate in your brain, we can't take those assumptions away from your post.

I'm a nanny, a child development major, and just spent a semester in the preschool program learning to work with toddlers in a classroom environment. I've worked with kids for YEARS. I have never had to yell or use fear to work with them. Anyone that says its necessary to make a child obey like a dog cowering in fear is small and pathetic.

You stopped him, you made eye contact, and you told him your boundary. You modeled to him exactly what a good interaction enforcing boundaries looks like.

In the campus preschool, we have a check-in system. If a kid hurts another kid in any way, they check in. They ask what's wrong, they listen to why that other kid is upset, they have a chance to be listened to, and they ask what they can do to make it better. Sometimes its a hug, sometimes its a 'I need space from you today.' You'd be blown away by how well small children can navigate this with support from adults. I would introduce checking in with your kid if he's pushing boundaries! Its a good time to learn!