I tried to find an answer

[hlthe2b]

4. Sterility implies nonviable or absent sperm. The reasons for male dogs (or humans) not "breeding" are not

limited to sperm issues, but include low testosterone levels driving mating desire, behavioral causes influencing lack of desire with a given female, physical problems (e.g., pain in copulating from injury or osteoarthritis/hip dysplasia or pain to the penis, urethra, etc.)

Parvovirus has been shown to cause infertility, abortion, and stillbirths in female dogs. Not specifically in male dogs, unlike some infections (e.g., brucellosis) or bacteria causing prostatic infections.

But, "sterility" as it sounds like your veterinarian used the term is nonspecific and it is more likely the dog had low sexual drive from hormone insufficiency--even if he had testicles. Or, perhaps he just did not encounter a female in heat.