...........a new pc............without hard drive..........I put his old hard drive in the new pc.
Sorry to disappoint you both but it's unlikely you'll ever get it to work. The new PC will have a different hardware configuration and that's what's confusing the Windows 7 on the old hard drive. Even if you do get it running, Windows activation will fail because your licence prohibits what you're trying to do and it'll stop working.
You need to buy a hard drive and a copy of Windows. You can connect the old drive to the PC with a USB adapter so your son can access his personal data such as music, videos and photos etc.