I generally prefer jerseys not to be signed, but I might think differently in your situation with the use in a World Series game that you and your son attended. Like someone else said, I don't like the idea of altering the appearance of what was worn on the field. That's why, if a baseball jersey is to be signed at all, I prefer it to be signed down on the tail next to the tags, which is an area that's not visible on the field. You could have him sign his name and "Worn in 2013 World Series," or "2013 World Series Jersey," or something like that (hopefully not so huge to be distracting from the overall jersey). Just one more option to consider. Whatever you decide, I think it's a great thing to be able to give your son. Enjoy it together.