You're assuming he is leaving for tax purposes. If he doesn't live in the United States why would he retain his citizenship? He could be denouncing it for entirely unrelated reasons, someone stated below that to get citizenship in Singapore you must be a citizen of only Singapore. Nowhere has it been confirmed, this is all speculation. He hasn't just suddenly "split", he left the US almost 2 years ago. He already paid taxes on hundreds of millions, I really doubt that he hasn't paid his "fair share".