He calls him Akihito from the start:
http://www.mangago.me/read-manga/you_re_my_loveprize_in_viewfinder/mh/v01/c001/28/
And as LadyLigeia points out, only calls him Takaba after that when they're around other people.
When other people are around or when he references Akihito to someone else, he calls him Takaba still:
http://www.mangago.me/read-manga/you_re_my_loveprize_in_viewfinder/mh/v07/c041/9/
Around what time did Asami start calling Akihito by his first name? Was it when they moved in together? 'Cause I just re-read the Temporary Paradise extra that takes place after the HK arch, and Asami was still calling him 'Takaba' then.