That little son of yours is more than one child. Remember he's part you.
I know exactly what you mean by this. It's happened to me here and there. I hear the sound of a little kid crying - not necessarily my own kid - and I somehow identify... pain with pain. If someone else is in pain, sometimes I have a keen sensation for it. It makes me a little sad, remorseful, whatever, and then it passes. And so with that son of yours, remember that there will be smiles and laughter too.
Everyone has emotions. No one is Mr. Sunshine all the time, although we try to be. So we have to accept some highs and lows, but never, NEVER dwell on the lows. Keith was right. This too shall pass.