In water that cold, and a child that young, it’s actually quite likely that the boy will suffer no permanent ill effects. It’s called the mammalian dive reflex.
About 20 years ago a young boy fell into the icy waters of the Red River in Fargo, ND; he was there for 15-20 minutes survived and recovered nicely.
I did not know the name, but I did know there were examples particularly with young children surviving in similar accidents and coming back to normal. So even before I found the article I knew there could be a good result.