Its just more personal and you feel more connected.
I'm sure he is trying to prove to himself that he can make it on his own.
Right now the best thing you can for him is be his cheerleader.
ADHDers miss alot of social cues, things like body language and tone of voice. It also ties up hyperactive hands that want to be doing something, anything, now!
You might want to try cutting down on the phone and invest in Skype, a webcam, or something along those lines.
I constantly tell myself i can do it otherwise i get depressed, and furthermore he may feel isolated from the crowd, lonely at times knowing that he's not like everyone else, and so calling and checking up on him IS a good thing, its best if you space out the calls to once every 2 or 3 days.