An answer that hasn't been given yet but that ties in with the 'he's unchecked me in his friend's list' is...well it's embarrassing probably to be caught showing offline when you are online, to friends. If this is indeed the case (and it's not just a glitch) how about just respecting his need for privacy for the moment? It's hard to want to do some things in SL when you have a large Flist and get a lot of IMs. Sometimes we're feeling down or just want time alone...just as in real life, these things happen.