Also - re: the diaper rash. It's possible he was neglectful and didn't change it. But be careful about assumptions. My son used to get wicked diaper rash anytime he drank orange juice. It's possible that they gave S2 something that didn't agree with him. You might try inquiring - nicely, by email so it's documented - about the diaper rash. Attach pictures and just ask "Hey, S2 came home with this wicked diaper rash (see attached photos). Did you perhaps feed him something different or use something different on his bottom that he might be reacting to? I'd like to make sure I avoid that thing in the future whatever it was." That way you're not accusing him, but it's documented that he had the rash and that you have the photos. Puts him on notice to be more careful without putting him into defensive mode (or at least giving you plausible deniability if he does get defensive).
Really, I think if you ask him to take S2 every Saturday night he'll refuse because it will dawn on him that this is date night - and who might you be going out with? A little bit of aikido - use his own weight to throw him.
