How about something like what you wrote here? The truth is, he won't want to read one more Sympathy card, so maybe you could simply write him a note and in it mention that you were pleased to have had the chance to reconnect with family recently, his in specific, and hey, how in the world were you connected anyway? And that his mom was a shining spot in your life and you are going to miss that. Then keep in touch over e-mail, or suggest getting together on a specific date, near a specific occasion or in a specific month.

Eh, just a thought.