Melissa, I think you made a very wise decision tho a hard one. I'm very sure there's a lovely snake out there for your son and family to care for. I would suggest that maybe before choosing another one you look at the options, ask some questions here and make, with your son, a really informed decision.
It was also suggested to us once we decided to go with a specific snake to set up it's home a few days early to get everything sorted out as far as temps & humidity, hides, water dish, etc. Then once we got our snake to let it settle in and not handle it or bother it at all other than to remove feces/urates and refresh it's water for at least one week. That's hard to do but pays off big time when it comes time to offer it's first food after that settling in period.
We followed all the advice of this forum and Melissa it's paid off wonderfully! We are thrilled with both our snakes and they fit our family like a glove (but not a snakeskin one! LOL)
Best of luck with the talk with your son and try not to feel bad, you really were given terrible advice by that pet store employee.
~~Jo~~