My take on that is that when he is with his loved one, he feels free, careless as in without any care in the world (in a song, it's easier to say careless rather than that mouthful I suppose) and he believes in himself as opposed to being unsure, full of doubt etc etc when he's NOT with his loved one. This is because the loved one (hey, could be his son!) gives him that belief, that faith in himself that he can leap off tall buildings in a single bound - oops, sorry, that's Superman - and do things he didn't think he could.
Enuff said!
Kelly
It's concerning the line - "When you are with me I'm free...I'm careless...I believe"