This is an interesting question, stem kid. Does this happen because he doesn't read the directions carefully or because he wants to finish the test quickly and doesn't write down enough information?
I think most of us are guilty of this occasionally. Sometimes I still read directions too quickly and miss a couple things!
. This isn't my area of expertise, but I can try to give some suggestions.
For reading instructions properly:
Since your son is only in 5th grade, does he care about his grades yet? If he does, and he is getting penalized for missing pieces of his projects or not reading instructions, perhaps sit down with him and the teacher and explain to him clearly WHY he is getting points off. If he sees how easy it is to get better grades just by reading the instructions carefully and checking things off, maybe he'll see the importance.
The next time he gets instructions of some kind, sit down with him and read the instructions together. If the criteria isn't listed in a checklist or bullet points, have him write out what he needs to do in checklist format.
As he does the assignment, remind him to check the list occasionally and verify that he is going in the right direction. When he thinks he is done with the assignment, go over the checklist and make sure you can check everything off.
For giving more details:
If the teacher doesn't give a list of criteria, have him talk to the teacher and give specific instructions on what is needed to get a good grade. If it is a project, have your son meet with the teacher regularly to get feedback before the project is due.
You could also have your son brainstorm other details he could add to his answer. If he's good at remembering facts, have him write down everything he remembers that relates to that question. Again, sit down with him and the teacher to see what the teacher is looking for. Is there a pattern to what he leaves out?
At first, he might need your supervision, but hopefully he'll understand the method after a while and be able to do this himself.