My 5 Year old's embarresed!?

So last week, my 5 year old son was doing a concert for school. They were all happy and singing until he decided that he needed the toilet there and then. He couldnt hold it and peed himself. Everyone was laughing at him so i wsa told to go on the stage and take him home. He is so embarresed and crys whenever someone mentions it. Not only that but his older sister whos 7 teases him. I have grounded her for it but she thinks its funny. He was wearing jeans and everybody could see. Any Ideas to make him less embarresed?