First of all - HAPPY BIRTHDAY POM!!!!!!!!
Now, onto your question. I guess the kind of letter you send to him all depends on what you feel you must say.
If you feel comfy keeping it short and to the point without going into detail (as obviously he knows what he did wrong) then that is perfectly fine.
If you feel there are things left unsaid that you need to get off your chest, then by all means, say them. It could be now or never you know?
The important thing is to go with your gut instinct on this and do what YOU feel comfortable doing. Maybe you could write 2 letters.....one short and one longer. Read them over, see how you feel about them and choose from there. Give yourself a couple of days before mailing to take a break from them and then reread them so you are in a better frame of mind
Wishing you well Pom!

sabby