The shaft size of 5MM sounds quite large for these smaller planes. Mist have shafts from 2.5 to 3 MM. Can you measuer it"  3.5MM is 1/8".  Also does it have a long shaft that the prop mounts in or just a short on near an inch long.

I found many of these have the same mounting. Also, is your motor body turning which is an outrunner or does the body mount to the bracket and just the shaft turn which is an inrunner.

Does the ESC which is the little flat thing between the motor and the battery that the wires come on go from say what amperage like 15A or 20A etc.  Before you can search for a motor you need to know more on what you are looking for. How large in diameter is the propeller? 

Sorry for these basic questions but they will help your search. Most motors I have ordered come ship from Asia and take a moth to get here. I just got my last one yesterday that I ordered from Hobby King in early January.

Several places like Radical stock quite a few. ask Dave at  Radical R C <[email protected]>. He is most helpful on things like this and has a good inventory too.

I hope this gets you on the right track and your son back in the air.