No, you can’t really use “something” and “somebody” interchangeably in this context. He thinks he is something → he thinks he’s important or valuable in a general, abstract way. He thinks he is somebody → he thinks he’s someone important or notable to others. He thinks he is special → he thinks he’s unique or different in a positive way. So each one has a slightly different nuance. Using them interchangeably would change the meaning of the sentence.

In the American vernacular, they can be used interchangeably. They all express the idea that he thinks that he is "better than." I, personally, use them interchangeably.