Probably is an undescended testicle meaning it is up in his abdomen. It can be fixed with surgery not really life threatening. He is however more prone to certain diseases and conditions later in life. Doesn't mean it will happen he is just at a higher risk. Talk to a doctor about this and make sure he knows this when he grows up so he can take the correct preventitive measures. It is really important.
You don't mention your son's age, but working with a pediatrician, an operation could be set up to search for the missing testicle. However, quite often males are born with one testicle and live a normal, fulfilling life fathering children and so on. The single testicle is enough to provide for all necessary bodily functions just as a single lung does for some people. I know of one married man, in his early sixties now, that was born with a single testicle, and he has had no problem living, working, going to Viet Nam in the Army, fathering a child, and so on. If there would be any problem, it would be psychological in nature. A good pediatrician will be helpful here and answer all doubts.
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