[tjic]

Read the letter, specifically this bit:

Mike, you know better than many what an unhappy home is and what it can do to others.

Seems to me he's saying "I've made mistakes and learned from them".

[brucejaywallace]

That's what I got from it. It takes a sense of humility and ownership to admit those mistakes and be able to reflect on that and produce very useful advice to his son about it.