Can someone explain why all the spermatogonia produced from a single sperm mother cell have identical DNA?
The spermatogonia are diploid stem cells formed by mitosis - so they have the same DNA as a somatic cell.
After that they differentiate into 2 types, A and B. Type A spermatogonia continue to divide mitotically to keep the supply of stem cells going. Type B are called primary spermatocytes, and go down a different developmental pathway, dividing by meiosis to form haploid spermatids.