Sounds like you were blocked.
Blocking will remove you as friends and they will only be able to see what you allow the public or friends of friends to see.
When you block someone, it does not delete messages they have sent you.
When you block someone, you block their account, not their name - so if that account changes it's name, it will still be blocked. They can change their name, address, photo, whatever they want on that account and it will still be blocked.
As far as I can figure, the name that you see listed on your messages changed for 1 of 2 reasons:
1) they changed the name listed on their account - the message will always display whatever name is currently associated with that particular account at that particular time
2) Facebook F'd up and is acting wonky or just doing something stupid that cannot be explained by any rational logic.
either he's disabled his account or blocked you from the looks of it.
For problems logging in email HELP@GrownUpGeek.com