I think you may be reading too much into Part 3 of the form. It relates only to any payments that you will receive on your son's behalf from Social Welfare - not to any other financial affairs or assets that he may have.
It's a standard form that is issued to thousands of people annually. And if you don't complete it as requested, then SW won't be in a position to discuss your son's application and associated matters with you. Is there someone else who could deal with Social Welfare on his behalf if he's unable or unwilling to do so himself?