agree Twigg, Impetigo is extremely catchable and can be very nasty.
And I can't help feeling, even issues of infectiousness aside, that a child with an ear infection, strep throat, and impetigo, needs to be at home, not dosed up and at a party. They will recover all the quicker if they have been allowed to rest at home IMO.
I understand he had been with other children all week - but that was before the diagnosis of strep throat and impetigo; once you know that what he's got, then it's inappropriate to send him to a party!