وَكَانَ مُخَيَّرًا فِي الْبَاقِيَةِ، إِنْ شَاءَ أَذَّنَ وَأَقَامَ، وَإِنْ شَاءَ اقْتَصَرَ عَلَى الْإِقَامَةِ
Translation: For the remaining prayers he has a choice: he may give both adhan and iqamah, or he may give iqamah alone.
Mukhayyar means having a choice between two permitted procedures. In the subsequent prayers, repeating adhan is not required in this formulation, while iqamah is still given.
For example, in a sequence of missed Dhuhr, Asr and Maghrib, the first takes both calls; each later prayer may take iqamah alone or both.