I would suggest that you and your son make the decision together. I would encourage him to lurk around this forum and read the shave Wiki's and to check out Mantic on U-Tube. Just like most of us on this forum have done.
He may just want to start out with the disposable and a can of shave cream or he may want to try the traditional wet shave method with the DE razor and a brush and making his own shave cream from the beginning. What ever decision you make be sure that it is an educated one.
Also I would suggest that you take it slow and easy with him just like I did with my own son. He won't need to shave every day at first.
Here are some facts that I have learned about shaving:
1. If you are going to shave your beard with a razor, you are occasionally going to get cut. That is a given. My son became discouraged because he cut himself so often. I just kept reinforcing him and today he is a very apt shaver.
2. Shaving is an art, one must master the techniques in order to be a good shaver. Learning how to prep the beard, how to make a good lather, how to handle the razor, and learning the techniques are somethings you can't learn over night. Be patient and determined
3. Lastly, what is taught here is the manly art of shaving. It is one of the things that sets a boy on the road to being a gentleman. Learning to shave is something that someone never forgets. Your time spent with your son in this endeavor will produce precious memories for him the rest of his life. Good Luck to you!
"Two roads diverged in a wood, and I—I took the one less traveled by, And that has made all the difference." Robert Frost