Our current cats all like milk - all our cats ever have liked milk. Cassius liked cream better, if it was available. None of them have been ill with it. Funny though, they only drink shop milk - none of them would drink our raw milk when we were milking.
Cat's don't drink all that much water, I don't think, unless they are on dried food.
I'd just go with the flow - if a wee saucer of milk makes him happy and doesn't make him ill, go with it.