First of, I'm glad to know that your boy got such an amazing father to take care of him.
Now let the focus establish on what works great and skip worrying about what seems gone but is not.
Your boy is born without any problem and that's a must. Thanks god he is not born with any testosterone deficiency or even without any testicle! What is left will take over (usually testicle will enlarge, sometimes as big as two testicles, furnish as two, keeping the normal testosterone level just like he had two) especially since he is at a young age, it gives his body way enough time to balance everything and aknowledge the remaining testicle to work at necessary potential.
He has all success within him towards all manly activities and beyond average.
You can always check his testosterone level, especially at the common peaks to make sure everything is normal. In the worst case, hormonal therapy is a piece of cake these days and won't bother someone who needs it.
It will be a ment to be experience for your child that will get him a beyond strong and solid self-confidence, he is capable of everything.
After getting over it, only adventages are to embrace ( from esthetic to performance).
It is true that some people will have a rough time going trough this, experiencing fear, depression and stress towards their situation but it will always be about perception.
Just remember that he is normal and beyond.
Other things influence our testosterone levels such as moods, nutrition, social relationships, exercise, stress, valued experiences, age, etc... It's really not all about testicle(s) here.
always, wait to worry
If you want to know about my experience, my personal pros and cons, you can always private message me!
Cheers