R.S.
You have to take him to have a new cast put on. The pee inside can eat away at his skin and cause an infection.
My daughter broke her tibia at 11 months. Our pediatric orthopedist told me if pee or poop got inside the cast that I was to bring her straight to his office during office hours or to the ER on weekends. From how he stressed this I understood it could happen pretty quickly...the skin infection.
So go ahead and call the office first thing in the morning and check...but a new cast is probably in your future. Sorry!!