If you don't want to do a Goofy Greet twice, you could possibly have it laid tight on a clipboard or embroidery hoop for signing, then have your son pop it on over his other shirt for the picture.
P.s. I am a scout leader and the girls often do fabric crafts. Sharpies are permanent on cloth, keeping their brightness especially if you dry 10-15 min on high prior to washing, to set the color. i do find that a true fabric marker is less likely to "bleed", although sharpie doesn't do too badly on 100% cotton.