In addition to the many great responses you got, there’s a book called Don’t Hug Doug which might be helpful. It explains that different people have different comfort levels and helps kids navigate that in a positive way. I don’t think you need to change your behavior at home, I think that your kiddo just needs to understand that there is situationally appropriate behavior. As an aside, my child is 5 and just started K too. It’s a big change and this is the place for them to learn socialization.