You say, "for the first time" -- since how long? If he has long had the lack of descended testicles, it might definitely be something to discreetly consult a physician about, or else it could be much more embarassing to let it go until he gets into sports & starts showing at school. That could be a horrific embarassment for him and psychologically-harmful taunting by his peers.
You would have to determine if the testicles are present during warmer conditions as the testicles will retract when cold. If they are available then leave things as they are.
Retracations of the testicles is normal during colder conditions.
As for accidentally seeying your son nude, this is not uncommon. My 17 year old son walks from the bath to the bedroom in the nude if there are no visistors in the house.
At the age of 9 my son would not have know what is considered to be normal once it comes to developent of the genitals. If you don't feel comfortable dealing with this matter, your son, chidren should have a medical twice a year.
All the best.
Ron