To me, the most substantial concept of belonging is that of the intraspective, or 'personal'.
Both Romulus and Raimond are stark examples of how 'personal belonging', or in other words 'self identity' is the foundation on which you build your external visage.
For Romulus, the story is all about his struggle to maintain his identity. The book is forth-most a retell of what Romulus sacrifices in the name of conformity and acceptance, but even more so; what he refuses to alter (the pretence of pride, morality, ethics and value most significantly). Romulus is in constant battle with his own identity throughout the book, the social, cultural alienation/belonging are mere stimuli which help to mould his perception of his roll and position within society, family and most of all; self.