Judy, serum testosterone level can be measured by any of our reference laboratories but it's not recommended to do so. Even if found to be low, exogenous testosterone (that supplemented by injection) may suppress release of GnRH and LH, thereby interrupting normal testosterone synthesis by the interstitital cells of the testes.
The first step in investigation of infertility in the tom is collection of the reproductive history and a thorough physical examination. Various reproductive disorders causing infertility have been described in the tom cat such as testicular hypoplasia and persistent penile frenulum. Particular attention should be paid to examination of the penis (position, size, ability to extrude from the prepuce, presence of spines), and testes (size, symmetry, consistency). Ultrasonography may be a useful tool for investigation of testicular abnormalities. Collection of a minimum database (complete blood count, serum chemistries, complete urinalysis, feline viral testing) is recommended. A vet should obtain information on AJ's housing, diet, (including nutritional supplements), and medications (both prescription and nonprescription).
Because AJ appears to have normal libido, you should witness matings (in person or by video) to ensure that normal events are taking place. For example, if no postcoital reaction is seen from the queen, intromission was likely unsuccessful. Semen evaluation may be a valuable diagnostic tool for tom cats with normal libido, where other causes of infertility have been ruled out. This must necessarily be done by a veterinary specialist theriogenologist who has the specialized equipment for collection of feline semen.
Please respond with further questions or concerns if you wish.