We use Aeroguard to remove permanent marker, works really well on hard surfaces, I am not sure about fabrics, but I would look at testing it on a piece of fabric and then perhaps making a paper towel damp with the aeroguard and blotting and see what happens, I wouldn't rub it as it may spread it.

And buy your DS some Crayola washable markers, DD1 had drawn on so many clothes with those and so glad it has come out