Would he also have to pay taxes in Singapore? If so, I can see how this move would be acceptable.
Even if not, I don't think many would find this unacceptable if he had done this before the mention of the IPO. It's possible that he has been considering the move for a while, and hasn't bothered to go through with it until a significant chunk of his income became at risk.
I'm not a lawyer, but this seems plausible. Singapore doesn't have a comprehensive tax treaty for the USA[1], which would otherwise prevent double taxation.
Even if not, I don't think many would find this unacceptable if he had done this before the mention of the IPO. It's possible that he has been considering the move for a while, and hasn't bothered to go through with it until a significant chunk of his income became at risk.