What sometimes does happen though is that the testicle(s) moves back up (retracts) into the inguinal canal in response to an exaggerated muscle reflex, fooling parents. More commonly, with "retractile" testicles, one is able to bring down each testicle with careful palpation when the child is relaxed in a warm room or by placing him in a tub of warm water, which helps the testicles make their way back down into the sac. This examination can be made easier if the child is in the squatting position. Often more than one examination by you and/or his doctor may be necessary to establish the correct diagnosis.
A "retractile" testicle usually finds a permanent position in the sac during puberty and has none of the complications commonly associated with a true undescended testis. If there really is any question, I would let his pediatrician see if he can palpate the testicles, documenting that they are normal size, position, etc.