Hi Gabbysdad,
First, you should enter your license key into your forum profile so that you have access to the Missions board. That is where information on missions is to be found.
There you will find bold blue topics that explain how to search for information on specific missions and how to start a topic to ask about a mission if no topic exists for that mission.
If your son has already registered the license key, you will need to log in on his account.
In order to help you with your current problem, I need to know the exact title of the mission. Because there are hundreds of missions from various sources, I also need to know where you got it:
Installed with the game and accessed via Play > Mission ...or
Downloaded via Play > Custom Missions. ...or
Downloaded from the Creators Forum ...or
Downloaded from a forum post as a ZIP file
It is also helpful to know the starting environment and the creator’s name.
Regards,
Marty