Alcohol works pretty well (as I used to have to remind students about half a jillion times when they would pick up a sharpie to label testtubes or vials with alcohol-preserved specimens or alcohol extractions of chlorophyll in them). It doesn't take sharpie off of skin
quite as fast as it takes it off of labels or glassware -- one drop or splash and presto you have NO way of telling what was in there
-- but with a bit of rubbing it will come mostly off and a day or two later the rest will have disappeared.
But I am also with those who think that making him wear it for a while might be instructive
I remember when I was teaching summer riding camp at a hunter barn, many years ago, and two of the boarder kids (with expensive hunter ponies) used livestock crayons and 'washable' hairdye to write graffiti all over their [white!] ponies for a costume class in the camp horse show. The day before a weekend-long A-rated hunter show. You can pretty much figure out how that one worked
Their trainer would not let them go to the show as he did not want to be associated with sending something like that into the show-ring -- and also I expect he figured that forfeiting prepaid entry, stall and vanning fees would cause the kids' parents to motivate them never EVER to do such a thing again.
The rest of us thought it was pretty funny though
Good luck with the sharpie,
Pat