That's not entirely true. He could have testicular cancer, which is not uncommon in unneutered dogs....
The growth was described as a growth on his butt. There was no mention of an inflammed or swollen testicle. Therefore there is no foundation for testicular cancer, unless it is to be secondary as a result of metastasis.

It COULD be a histiocytoma, but it could also be cancer or another type of growth.
Yes it could be other such as Hemangiosarcoma, Hemangiopericytoma,
Mast Cell, Lymphoma, Fibrosarcoma, Rhabdomyosarcoma, or even Leiomyoma and Leiomyosarcoma. And yes the only way to definitively know for sure is for the Vet to send a needle aspirate (or a biopsy) to Pathology. Some Vets are lax in doing so, and that is why it was suggested.