I heard half-sack in a TV series before to describe a man with one testicle. Is there any single word to describe that?
Monorchism is the word to describe the state of having a single testicle. See Wikipedia. You could call a person with one testicle as a monorchid individual.
The Greek root -orchis stands for testicle.
Monorchism is a correct word, but cryptorchidism or cryptorchid is probably the more commonly used/heard word since it is the most common birth defect of male genitalia. It is the absence of one or both testes from the scrotum. With cryptorchidism the testicle may be within the abdomen, undescended — but it is absent from the scrotum. A boy or man with one undescended testicle could be called a unilateral cryptorchid. (This condition occurs in veterinary species, as well.)