It's hard to actually say he lost interest or seeing someone...
Have you tried telling him how you feel? He needs to know how you feel.. If he doesn't understand then I don't think you should still be with him.. Because he doesn't put enough effort to this relationship.. My advice.. call him and ask why he never calls you.
Maybe he is just busy
Could there be a good reason he isn't calling? If there is there's probably nothing wrong in the relationship. If there isn't a good reason then something is wrong and you need to talk to him about it and tell him how you feel.
Usually if I really like someone I can't get enough of them so couldn't go that long without calling or seeing them :)