"man up"
and talk to him.
It's not some big shameful secret. Practically and hormonally it is a non issue, as you have no doubt been told he will mature sexually as normal and have a full fertile sex life ahead of him (with a slight chance that he may not produce such high quality sperm but that isn't a great problem as a rule).
He needs to know that all will be well even though he only has 1 testicle- if you don't tell him no one will and if there is an atmosphere of "we don't talk about that" he may well stew quietly and get really worried.
IMHO talking about "embarrassing stuff" is best done in the car when you and he are alone- no eye contact, no interruptions LOL .
Make sure he knows that actually he his none the worse for having 1 testicle, and particularly, from a practical POV he knows he needs to take contraceptive precautions as seriously as anyone else or there WILL be unexpected surprises.
If he were my son I'd say that's how you are- a one testicle wonder or similar, and yes you could have a "falsie" if it bothers you (I would think that would be done at the end of puberty) and that is your choice. However why don't you wait and find out if actually you just say to your GF "I'm a one testicle wonder due to an op as a baby but I see no problem with that" an umm "carry on regardless".
Let's face it he wont be bullied at school for having one testicle- they all shower in swimming trunks if at all, and if his sexual confidence and self esteem depends on the testicle count then maybe he's got more to worry about than putting that cosmetic issue right?
I might also point out the current breast implant worries as cosmetic enhancement isn't without risk.