Yes, it is permissible for him to make up the days separately, because Allaah says (interpretation of the meaning): 

“and whoever is ill or on a journey, the same number of days which one did not observe Sawm (fasts) must be made up from other days”

al-Baqarah 2:185. 

He did not stipulate that the days be made up consecutively. 

And Allaah is the source of strength, May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote. 

Standing Committee for Academic Research and Issuing Fatwas 

Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Ifta (10/346).