I dunno about you but I find some satisfaction when I know I've handled something okay. Stuff of all kinds happens in a lifetimes but I think it's how we deal with it that counts.
Just a suggestion but why not remind him that although he's starting back at day one it doesn't mean all the effort he has put in over the last 2 years has disappeared - how can it - it was and is still real?
When it comes to explaining stuff - to anyone of any age I try to think first what it is I really want them to understand, and then how I can help that understanding with what I say. It might be helpful to remind them their Dad is a whole person not defined by any one action - that goes well sometimes with explaining illness, forgiveness, learning self forgiveness, motivation and hope.
Good luck and best wishes to you both.