Don't panic whatwhat - you did the right thing.
My wife was head of a primary school, and always talked of concerned mums asking about bullying - intentional or not.
From your perspective, you want to do everything to protect your child - even if the bullying is not intentional, it still does damage, and needs to be stopped.
Teachers have masses of experience in this area, and will know how to deal with it in every situation.
The teacher would never go to the perceived bullying child and say anything like "I've had Mrs whatwhat in here saying you are bullying whatwaht junior, so pack it in....!" they are far too clever and sensitive for that.
Your teacher may have a quiet word with the child involved about being nice to everyone, and how not everyon plays like him - along those lines. If the bullying persists, they will make the message a little stronger, and so on, until it sinks in.
So give the teachers a few days to implement this practise, and keep discreet tabs on your son - asking how his school day was, without any mention of behaviour - he will tell you if its still going on.
If you think it is still happening, have another word with the teacher, who will be happy to take on board your concerns.
It's part of growing into the school society, and it's not always easy for sensitive children - I've got one, and I was one so i know!
With regular co-operation with the school, you'll be able to steer your child through, and remember, it's all new and upsetting for you, but teachers do this for a living, so they will sort it out - promise!