This has happened to me more than several times! That is why it is so important to remove these dewclaws at birth or a few days after. I had a young german shepherd mix that when she was spayed I had her dewclaws removed as they were double ones! It was quite an operation, as the attachment becomes bony after puppyhood.
If he will let you, trim the nail back, put some antibiotic on and wrap it. I had all of my dogs treated by a vet, and professionally bandaged. Otherwise if you wrap it too tightly, it will cause serious problems, too loosely, and it will come off.