He should see a Neurologist. There are diseases, like Parkinson's disease, that could cause these symptoms.
Boy, that sure sounds like a form of a seizure, doesn't it? I had a seizure (first and last ever, hopefully, from water intoxication last summer) and it started exactly that way. I was seated, and frozen and my kids were trying to get me to react, and immediately called my husband on the phone and he was talking to me (while my son held the phone) and I could hear and understand, but couldn't move or speak. Then I went into an actual seizure. Your uncle's case sounds like what used to be called absence epilepsy, or "petit mal" seizure - except he's aware of what's going on while he's "frozen".
Best wishes. It's very interesting that his mother had this, too.