Here is an alternate take on it which might help:
>>aya.m kumaaro jaati-aadisampanno, so jaatigottakulava.n.napokkharataadiini
nissaaya maana.m vaa mada.m vaa maa akaasii"ti<<
This boy has been endowed from the time of birth (or with birth, etc.), on
account of his birth, family, clan, appearance like that of a lotus, and so
on, let him not have (or show) conceit or arrogance.'
I don't take kumaaro as a vocative, and I don't believe the context is of
him speaking just to Rahula but to a group.
I prefer exhorting.