I'm sorry but that's forgery and you can't and shouldn't do it - with or without your boss's permission.
The fact that you're able to sign his name well is another matter - I can do things like that too!
You'd be better and on the legal side of everything if you simply sign your own name and put pp against his name which is presumably in the signature block.
Other alternative, get your boss's signature scanned into your PC and then you can add it electronically to letters when he's not around to sign them.
BUT DON'T FORGE HIS SIGNATURE - THESE THINGS HAVE A NASTY HABIT OF COMING BACK TO HAUNT YOU.
Jackie
www.iqps.org
Peer Moderator 
