One day more probably won't make a difference.... they will probably diagnose it (which you already have noticed it), and tell you to see his physician for a surgical referral.
https://www.cornellurology.com/clinical-conditions/pediat...
Testicular retraction
This website shows that one of the reasons for an undescended testicle is not that uncommon, and disappears by age 13..... this may be the case with your son. This also may be the reason your doctor either has not noticed it, or has not mentioned it.....
I just found another interesting link.... the testicle may go back up due to cold, or physical activity...
http://www.patient.co.uk/doctor/undescended-and-maldescen...
Look at the part that talks about "retractile testes" and "Ascending testes syndrome" .... I think both of these will ease your mind.
I realize you are stressed and worried, but I really think this could wait until you could get in to see his regular pediatrician on Monday.
If you are really concerned, is there a nurse line for your insurance that you can call and talk to them?
Just curious... if this was brought to your attention a week ago, why is it now all of a sudden becoming an ER emergency? You could have addressed it during the week at the doctor's office.....