Chikusa's advice falls on deaf ears, which isn't unusual - but, as it happens, Ken can't ignore the warning because it's Chikusa who's giving him the food and drink. Chikusa is setting the pace, so if he feels Ken is drinking too quickly - which Ken will, if he's allowed - he can just pull the water away.

He'll get whined at when he does, of course. But Chikusa must be used to that.