You can try rubbing alcohol using cotton balls to get most of it off and then something like dawn dish detergent to bathe him in.
found some others:
Coppertone sunscreen and toilette paper took it off with ease. I applied it with my fingers waited about 10 seconds then gently scrubbed and wiped away all the marks with the toilette paper.
I used damp cotton pads then put canola oil on each pad. I rub and most of the sharpie came off. The light stuff came right up while the actual scribbles lightened quite a bit. Then I gave her a tubby and soaped off the oil. She looks pretty close to normal again. Plus, (I imagine) it's non-toxic.
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