Originally Posted by The Guard
The easy answer is "Clark is still a young man". Young people don't always know exactly what to do in a given situation,
Here's the thing about that scene. I don't think Jonathan's first line, "Get your mother to safety" is an accident. Jonathan wants Clark and Ma to get to safety. Like its his instinct to rescue the dog, it's just the man's instinct to avoid putting people in harm's way, even his (for all they know) indestructible son. And we don't actually know the level of Clark's indestructibility at that point, so it's arguable that Jonathan could well think the tornado might kill Clark, who, as we see, Jonathan considers pretty darn important.
He was younger when he had the instinct of saving the school bus and then go back for Pete Ross. This case being about his father deserves not indulgence. There's no way Clark would have stayed there.