How did Toomas Hendrik Ilves get US citizenship and where/when/why did he give it up? (former president of Estonia and curious character)

The question Do foreign heads of state or government have to give up a US citizenship if they have it? reminded me of the curious character Toomas Hendrik Ilves, "...an Estonian politician who served as the fourth president of Estonia from 2006 until 2016."

Ilves was born in Stockholm, Sweden; his parents Endel Ilves (1923–1991) and Irene Ilves (née Rebane; 1925–2018) fled Estonia after its occupation by the Soviet Union during World War II. His maternal grandmother was a Russian from Saint Petersburg. He grew up in the United States in Leonia, New Jersey, and graduated from Leonia High School in 1972 as valedictorian. He received a bachelor's degree in psychology from Columbia University in 1976 and a master's degree in the same subject from the University of Pennsylvania in 1978. He also received an honorary degree from St. Olaf College in 2014 in recognition of his relationship with the college. In addition to Estonian, Ilves also speaks English, German, Latvian and Spanish. By Ilves's own admission, he speaks Estonian with a comparatively strong American accent, on account of spending his formative and young adult years in America and Germany.

He shows up on Wikipedia's Former United States citizens

Question: How did Toomas Hendrik Ilves get US citizenship and where/when/why did he give it up?