Well if I understand what Tail Gland Hyperplasia is correctly, then it's caused by a gland and the shampoo is probably an antibiotic or clorahexadine (sp?) shampoo to help prevent infection.

Shaving the hair would probably help, but the shampoo, and neutering, would help more.

If he won't allow a full bath, try just getting a good soppy washcloth with the soap on it and try a tail bath. It might be less upsetting to your kitty than being in a tub of water.