I always thought that Asami calls him "Takaba" in public and "Akihito" in private. *o*
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.
He started calling him "Akihito" since the beginning of vol 7 (Yea he used to call him "Akihito" only on BED). Oooowwwch living together makes them closer and closer. Can't wait to see Akihito call Asami "Ryuichi" ლ(´ڡ`ლ) (Wait..if it happen what should we call him?????????)
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.