My MIL said one of her sons had an undescended testicle as an infant, but it came down on its own at some point w/o surgery; one of his sons had the same thing, but they did surgery when he turned 1 y/o and it hadn't descended on its own.
It sounds like you need more knowledge and information than you have right now, so I would suggest asking the pedi and/or the urologist the questions so that you know what is going on, instead of worrying about what is going on. It's very likely that your pedi is referring you to the urologist because s/he *has* to, not because you *need* to -- that is, there's probably nothing going on worth worrying about now, but if s/he doesn't refer out, then s/he is opened up for potential liability. So, my first call would be to the pedi's office and ask what is the problem with your son having an undescended testicle *right now*? If there's no problem now, which is very likely the case, then you don't need to rush out right away to the urologist, because there's nothing that can be done. You may further ask the pedi (and possibly be referred to the urologist, in which case you can call and ask him/her), at what age does it signify and/or cause a problem? How common is an undescended testicle in infants; how frequently do they remain undescended past 1 y/o; when does it become a problem if it doesn't descend on its own; when do most boys need surgery if surgery is necessary; and is there any benefit to bringing in your 3-m/o son now, if this corrects itself most of the time before 1 y/o, and there is nothing that can nor needs to be done now? [It's possible that there is some sort of waiting list, or some test that should be done, that would make it beneficial to get your baby seen now, but I have a hunch that it's not necessary, and there will be plenty of time to act on the referral in the future, if the testicle is still undescended by, say, 9 months.]