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.
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 :)