It won't cause any permanent damage as sperm production is continuous. As Dr. Veena said, heat will diminish sperm production, but only temporarily. Daily use of the hot tub would keep his sperm count low, but a few days off would bring it back up again. For a whole host of other reasons he should keep his hot tub sessions to less than 15 minutes.
Hi,
Thanks for writing in. Male fertility depends upon the semen analysis which determines sperm count, motility and morphology. Exposure to dry or wet heat can hamper sperm count and motility. Heat has an effect on sperm production.
As your child is still young you can educate him on this.
All the best