You are supposed to get a form from your pre med office (at your undergrad school) and have your prof fill it out. Then, he has to send the completed form and his letter to the pre med office. The pre-med office collects all your letters throughout your undergrad years. When you are ready to apply, the pre med advisor looks at all your letters and writes a "composite letter." That composite letter, which is kind of like a summary of all your other letters, is sent with the whole package to the different med schools. You are NOT supposed to touch/see/read any of these letters. In fact, schools won't accept anything from you and you'll have to sign a form at your pre med office saying that you waive your right to see the letters.
Go to the pre med office at your school and make sure that's the way they work. It varies a little from one school to another... Then, go back to your prof and ask him to rewrite his letter and send it to the pre med office. You can't use the ones you have in your hands.
Hope this makes sense. Sorry... it's late and I'm tired.