The main reason for the letter is for him to unburden HIS soul.

He has made the effort to do that.

O.k., But will that change the past?

Nope.

You can't go back and change things after you do something stupid.

I may have missed this but did he go to jail for driving drunk or causing you to miscarry???

To me it's far easier to seek forgiveness for driving drunk than causing someone to lose their child.

Let him stew in his juices.