S.P.
I wouldn't use Sharpie or ballpoint pen. If you go to a make-up supply store like Naimes, Frends or Cinema Secrets (in North Hollywood/Burbank) you can get temporary tattoo ink that is safe for skin and will come off with rubbing alcohol when you are ready. Just get a little bottle of ink and a fine brush and paint safely away any info you want him tattooed with :)