one hell of a friday

So the closure went well, everything got laid out and sorted. My ex and I are now on speaking terms and sort of friends. However......of course I still have small feelings for him and the fact that he has thrown hisself into a new relationship ( even though he's known this girl for years) stings a bit. He looks so happy though and he makes her feel safe and happy in turn. However she has sooooo many issues and baggage and has already cheated on him once. I don't want my personal feelings to grow for him ever again. And from seeing him like this I can now clearly see how I dodged a bullet because he has admitted to me that he wouldnt mind getting her pregnant if it means her abusive ex will leave the picture once and for all. 

This is in no way healthy and I want to be his friend but at the same time this is a situation that is really up to him and what he believes is best for him. He has already proven that he can move on from a girl he claimed he loved in a matter of days maybe weeks but then he's onto whomever pursues him or vice versa.

How can I be a good friend to him without making it look like I'm trying to butt in?