my son did that same thing a couple of years ago, i think he was 4. the drs office told me to have him drink some water and if that seemed fine to just watch for it to come out the other end. I searched for a week and never found it, so i called back and we went in for xrays.
within an hour of coming back from xrays the quarter came thru, but we have a cool xray of a quarter in his stomach!
sorry for no caps, i'm nak.
also, i sealed the poop in a ziplock back and to search for it, it really helped not to have to smell it while i was searching for the quarter.
hope you find it quickly and your son is okay too.