I would make an unscented, uncolored, 1% superfat(just in case your scale is off a tiny bit), low conditioning, but also not too high cleansing soap. Just give him as plain a soap as possible.
Something like this, maybe:
25% Coconut Oil(76 degree)
50% Lard
20% Olive Oil, pomace
5% Castor Oil
But, I am going to warn you that this might smell a bit like lard if you or he has a sensitive nose. I would strongly suggest at least a few drops of some sort of EO.