If I understand correctly, your son's HD (with his OS on it will not boot when wired into your old motherboard?
That is almost certainly because all the motherboard drivers are different from those your son's HD and system are configured to expect.
There is no quick fix. Had you made some headway to correct the errors you saw, it would probably have wrecked any chance of using the HD back in the original environment.
As I see it you must save all your son's data somewhere away from the HD, make sure you have the motherboad CD and then format and install Windows from scratch.