indeo, a couple thoughts. When you describe this to health care workers, I would avoid using the word "tripping". Tripping is pretty common in that age, some kids are more coordinated than others, and some just trip a lot, and it's not really a concern.
You go on to describe that he is collapsing from his knees, which is less common, and then he apparently has trouble when he gets up continuing walking due to his knee strength.