He could simply want to live elsewhere. If you don't live in the US, you wouldn't want to keep paying them taxes for no reason. I see no problem with renouncing his citizenship for tax reasons if those tax reasons are "I don't want to keep paying a country I no longer have ties to".

Taxes probably played a role in the decision, but it could easily have been a minor one rather than just "split[ting] for a tax haven".

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.