He spelled it the way it’s usually transliterated from Arabic to English. It was no mistake, and was meant to show where his sympathies lay.
Ameen is the Arabic version of the word Amen which has its origins in Hebrew and means ‘so be it’. Ameen is often said after a supplication (dua) with the spiritual meaning: “O Allah, respond to (or answer) what I/we have said”.
Ameen is often mentioned in the daily prayers of most Muslims after the recitation of the opening surah from the Quran.
http://www.islamic-dictionary.com/index.php?word=ameen
I would need to see some of his thoughts from before his son was captured.