His world has just shifted, after 9 years of being the only child in the family there is another whom he is expected to love and accept unconditionally. On top of that, we all know the fuss that people make of new babies. When the older child is a toddler everyone thinks of the displacement and they will get presents and extra attention, we forget that, it will be equally dificult for an older child. and in this case he is capapble of expressing how he feels.
I do not think he is spoilt, but I have a much younger sister and, in her case, again, it was not that she was spoilt, but she was the first consideration when anything was being arranged because she was so much younger than the rest of us. It is probably the same with this child, not spoilt, but used to the world just revolving round him.
Suddenly the world revolves round someone else and he feels alone and isolated.