I don't think it'd be rude to go knocking on his door (if you know it). It's just that it's a major anomaly to cease communications in such a way, isn't it?

NM and HN may be a low priority for him, and that's fair enough, and he can't be doing both forever, so you're not going to be saying "where have you been?" or "what's going on?" it's more like "is everything okay?"

A few people have been left high and dry, and if it wasn't for them, you'd probably back off completely.