No ---sitting in a hot tub for any length of time will not effect the future fertility of your son.
However, it does temporarily effect the sperm count of fertile men. I have had a customer that had to opt out of using a hot tub in order to ensure enough of his 'swimmers' were available to impregnate his wife, as they were trying for a baby.
On the other end of the spectrum I did view a documentary some time ago of some European (nordic?) men soaking their testicles in very hot water on a regular basis as an experiemental means of birth control. They did find that this 'hot water testicle soaking' reduced their count to almost nil- of course, this did nothing to prevent std's and wasnt 100% effective, and surely stung like hell.
