Verse 3 is an awesome thought. To realize that the God who created everything cares and guides our lives is beyond comprehending. After all, how can a single God control the lives of billions of people at the same time? As I have learned, if you can handle the first sentence of the Bible ("God created the heavens and the earth"), then you can more likely handle the concept that God is aware of the lives of billions of people. He knows the hearts of all and is willing to guide any of us who are willing to trust in Him to do what is best for our lives.
The term "son of man", usually refers to all humans. It is like saying the offspring of Adam and Eve is the people He cares about. Jesus Himself can be thought of as a "son of man" in that He was was 100% man and 100% God at the same time. He was the God-man, that is, God manifested in human flesh. David is speaking about humanity in general and not referring to the future Messiah (Jesus) in any way.
The comparison made here is that man's existance on earth is like a "shadow" of breath in comparison to a specific length of time. After all, shadows don't last a long time. As the earth moves, the shadow is gone. The shadow moves very quickly in relevance to an eternal time span. Time passes quickly and our lives pass quickly, especially in the context of the eternity of time as we could imagine it.
We would do well to consider the fact that God cares for us, despite how short our time span is compared to all of eternity. If we accept the concept God knows all things about the future as well as the past yet He cares for us, then we can draw upon Him and His knowledge of our lives. This ought to motivate us to use our lives in order to make a wonderful difference for Him. Right now, determine to do His will and make every day count for Him.