Complex sentence: Out of concerns for stigma, he began using the term posttraumatic stress, dropping the word "disorder", from the medical name posttraumatic stress disorder, which had become standard use in the armed forces, but was not taken up by the medical community.
Split into simpler sentences: Out of concerns for stigma, he began using the term posttraumatic stress, dropping the word "disorder" from the medical name posttraumatic stress disorder. His term had subsequently become standard use in the armed forces, but was not taken up by the medical community.