I think I would send a card, with just a brief message saying how sorry you are for his loss, the family are in your thoughts etc. it's very unlikely he would be offended or find it inappropriate.
I understand your feeling of awkwardness though, it's always impossible to know what to say when something like this happens, even if it's a friend, and doubly so if it's someone you don't know very well on a personal level. I'm lucky enough to have never been in such a situation, but I think I would appreciate any gesture of kindness, and knowing that people cared.