Concepts of Genetics 11th Edition by William Klug,Michael Cummings,Charlotte Spencer,Michael Palladino

Exercise 3

A man in his early 20s received chemotherapy and radiotherapy as treatment every 60 days for Hodgkin's disease. After unsuccessful attempts to have children, he had his sperm examined at a fertility clinic, upon which multiple chromosomal irregularities were discovered. When examined within 5 days of a treatment, extra chromosomes were often present or one or more chromosomes were completely absent. However, such irregularities were not observed at day 38 or thereafter.
What is the obvious advice that the man received regarding fertility while he remained under treatment?