When should my baby be able to fall asleep on his own?

My little man is 4 and a 1/2 months old, he still seems unable to fall asleep on his own, in order to get him to nap or sleep for the night he has to be on my . He wont take a pacifier. He nurses so often which I think part of that is comfort nursing and I don't mind it one bit but I am alittle worried that he will not fall asleep until he is nursing. The other night was the first time this was a problem, I went for dinner with a friend and my husband had to call me to come home because out son was screaming and would not take a bottle and he was so tired but he wouldn't just go to sleep, the second I got home he started nursing and just ped out exhausted. I felt so bad for leaving because I didn't know he would freak out like that but now I am concerned that I will never be able to leave for even a couple hours when it is close to nap or bed time because it caused him so much stress. Is this just normal behavior or what should i do?