Maybe Margaret Harris Peterson's Shadow Children series? I haven't read it, but here's the wiki summary:
The Shadow Children sequence is set in a country where the government has enforced strict population control laws in order to control overpopulation after environmental conditions have severely limited resources, particularly food. As a result, it is against the law for families to have more than two children; the law is strictly enforced by the Population Police, who are said to conduct raids specifically to find and punish families who have more than two children. The Population Police, due to its power, is usually corrupt and holds control over government policies. Unknown to the common public, the elite of society, known as Barons, have used their wealth and influence to live lavishly and circumvent the strict population and food laws. Due to these circumstances, the third (and subsequent) children of ordinary people are hidden and denied opportunities, often at great expense, and are denied the right to exist.
And of the protagonist, Luke Garner:
The protagonist of Among the Hidden, Among the Impostors, Among the Barons, and Among the Free. He is the third child of a farmer and has lived most of his life hidden in his house with no company aside from his family. He has two older brothers, Matthew and Mark. After meeting Jen Talbot, the third child and only daughter of his affluent new neighbors, the Talbots, Luke begins to question his status as a third child and the world he lives in. He begins a difficult path towards living outside of the shadows, beginning with leaving the house he grew up in to live semi-openly. He uses the alias "Lee Grant", the deceased son of a prominent Baron family, after leaving his home to live in a boarding school. When Lee Grant's brother Smithfield (Smits) visits, Luke meets the parents.