C.B.
Explain it to him that if he really wants to go to camp he has to do his part and go potty there too.
Seems kinda young at 4 to go to camp. But it still takes him understanding that you won't be there to help with potty time. "Big boys" sometimes gets their attention. If they want to be one of the 'big boys', they have to act like big boys do and this is part of it. Big boys go potty by themselves.
Gotta appeal to their egos sometimes. You may have to put off the camp, tho. Many systems have guidelines regarding being fully potty trained. But I mean, wow! S., you've got him trained for the most part at his age, and that's fantastic as it is! You're to be congratulated! Be patient, and realize that sometimes they just aren't ready for everything at the same speed of time that seems logical.