Hi Kate,
If your son is able to dictate his answers to the questions on the form and you are just writing/typing them for him, you should write it in the first person and note on the form that this is what you have done.
if your son is not able to dictate and the answers are yours then you should write in the third person - and also state on the form that this is what you have done.
In the first scenario above, your son would sign the form if he was able, or you would sign on his behalf. In the second scenario, you would sign the form yourself.
If CAB have any input to the form in their own words they should sign that section. It is more likely that they would give you information and advice that you would then write in your own words, so there would be no need for CAB to sign the form.
If that doesn't make sense, or you have any further questions, please come back to this thread