I believe it is the sperm that does not survive the warm environment, rather than the undescended testicle itself. But it sounds like time to see the pediatrician.
This procedure has to be done as soon as possible after birth cos the testicle cannot survive in the very warm environment of the body so thats why they need to hung out of it like they do. If it stays after the first year it might be an infertile testis or it might have to be removed. Its not too late to have the operation now. Don't leave it more time
Baby boys are normally examined at birth for an undescended testicle or testicles (unilateral or bilateral cryptorchidism).
In most cases the testicle or testicles will come down on their own by the age of 3 to 6 months.
As you child is now 6 years old and the testicle has not come down, it is not too late for you take him to see the doctor. The doctor may refer him to a paediatric surgeon for further evaluation.
Do not delay in making the appointment to get your son to be examined by the doctor. I am surprised this problem was not picked up at his routine baby medicals.
You may find the link below useful as it gives more detailed information about undescended testicles.
http://www.nhs.uk/Conditions/undescendedtesticles/Pages/Introduction.aspx
Let me know how you get on.
Best wishes.
Depending on the problem sometimes all that is needed is Hormone injections
It's important to have this checked early as it could cause infertility.
Talk to your child's doctor. I think it is better down than up, and it might be a simpler procedure to get it down than you think.