"joe". My best guess it that is a birth defect known as progressive congenital macrodactyly, where basically, starting at birth, the fingers grew rapidly, at a faster rate than the others, and then never stopped. If they did not amputate his hand I bet they continued to grow!
Credit: Wellcome Library, London. Single image from group of six on page plate depicting WW1 plastic surgery cases at the King George Military Hospital 1917-1918 taken by Dr Albert Norman, Scientific phototgrapher. Copyrighted work available under Creative Commons Attribution only licence CC BY 2.0.