Tricky. If he is just going to sign off on what you write, I would be tempted to do it. Focusing on the things that would make you a good match for the program. Good initiative, reliable, goal oriented, blah blah blah.
But if your boss doesn't know what youre going to school for that could be a sticky situation. Especially with the economy the way it is. Employers seem to let people go for minor things.
Is there someone else like someone you work with outside your company, a customer, or another company that works with you on a regular basis that could do it?