ive only just read through this post now, but just wanted to say im happy to hear that the two boys now get along.

dont be too hard on yourself, im sure you are a great mum. i think its wonderful that you care so much, i can remember back to my primary school days, that some mums were quick to assume the 'other kid' was doing something to provoke bullying.. this inst always the case. i think writing an apology letter is a great idea.