First look at the newspaper article about Joseph Dunn being discharged from the hospital after the shark attack. Note his address as "124E 128st".
Next look at Michael J Dunn's WW1 Draft registration. Note the same address. "124E 128st". This is obviously Joseph's brother who was with him the day of the shark attack. as mentioned Dr Fernicola's book. Also note his father listed as "James".
Taking that information. We have now found Joseph Dunn in the census. Note parents names as "James" (same as on draft registration) and "Mary".
Finally we have Joseph's birth certificate. Note "James" and "Mary" are parents (same as census.) Note birthday as March 19, 1902. Shark attack occured on July 12, 1916. This made Joseph 14 years old at the time of the attack NOT 12.