He just is now, having a bad experience at daycare, because of that kid.
Or, you can go there and observe too.
Teach your son, that he is fine.
Not all kids are nice.
Teach him HOW to tell the Teacher. Not just ask him if he did.
Give him the actual words he can use, to tell the Teacher.
3 year olds, don't have instant automatic skill, at wording.... so teach him and tell him the words to use too.
Role-play with him, to teach him.
GIVE him "skills" at handling wrong doing... how to TELL the Teacher AND you etc.
That it is OKAY, to do so. And the right thing.
So THAT, he is proactive... and not taught to be a "victim" about it, or wrong-doing done to him.
I taught my kids that, from 2 years old.
They speak up.
And are confident about it.
You gotta, role-play with your child.
You gotta, teach them the words/wording to use... to report wrong-doing or how to ask for help.
My daughter, at 3 years old, was Bullied by another kid. At Preschool.
SHE spoke up.... to me and the Teacher. Openly. AND had already told that Bully, "that is not nice, stop it..." and reported it, to us.
You ALSO TEACH YOUR CHILD, how to TELL other wrong-doing kids, to "Stop it..." , "I don't like that...", "I will tell the Teacher...", "You are not nice..... stop it..." etc.
You NEED TO, teach your child the words and wording about HOW to speak up... AND how to tell another kid, to stop.
Role-play with him.
So he does not learn, to be a "victim" and thus, gains skills in standing up for himself and how to tell the Teacher.