Vicki, the US State Department recently released an updated travel warning for Mexico and for the first time went region by region and named the good, the bad and the ugly, especially the places where foreigners go. You'll be pleased to know they had NOTHING BAD to say about Guanajuato. Have a look at our piece on the warning here: http://journals.worldnomads.com/safetyhub/story/82854/Mexico/Mexico-Travel-Alert
Similarly, traveller forums and blogs have nothing but praise for the town and the state. By the way it looks fantastic and deserving of its UNESCO heritage status.
You may have read from other posts on this site, I believe that tourists to Mexico are generally safe if they stick to the popular tourist destinations (and the town is one big tourist destination - for Mexican tourists!). The drug war is a war among criminals. Obviously bad things can happen anywhere, and your son should take the usual precautions, but generally I think he's going to have a wonderful, authentic Mexican experience. I've just put the town on my bucket list.
Phil from the safetyhub almost 8 years ago