well they have to find it. it shouldn't be difficult to locate if it is still there and not atrophied. an ultrasound should be able to easily locate a migratory testicle that decided to retreat back up the inguinal canal again for some crazy reason. if it is there it may need to be surgically anchored back into the scrotum where is supposed to live for the rest of its life. a testicle left in the abdominal cavity will not likely form sperm so your son's fertility will be reduced. children get very anxious during testicular exams and have very strong cremisteric reflexes (will temporarily pull a nervous testicle out of the scrotum and up into the abdomen) so consider having dad check it again when he is more relaxed at home. having him sit cross-legged may help pull it down or having him sit in a hot bathtub for a while may encourage this testicle to go south again. it can sometimes be felt in the groin if you know where the inguinal canal is but this is more in the realm of the medical provider. so start the hunt at home but see a pediatric urologist if you can find it.
my sons' left testicle seems to disappeared-his family dr. couldn't find it and it was there last year-he is 9 years old