This sounds like a value judgment that is placed on the birth parent. Most often being poor is associated with not having any money. A child may be better able to understand, 'She was struggling financially, and couldn't afford to take care of herself and a new baby. In some situations, age may be the reason for placement. Maybe a teenager wants to parent her child but she is being financially supported by her parents and they are not willing to support her child as well.
Age is often a small piece to a much larger puzzle.
Of course, as his parents, they need to tell him his adoption story, but only I can tell him how I was feeling at the time. I was afraid at the thought of being a Mommy. I wasn't ready. I was afraid to be a single parent. I was afraid of what my family might think if I told them I was pregnant.