dog wont walk on a leash

Question
I adopted a 4.5 lbs. 6 month old male dog who was not trained.  He is potty
trained now in the backyard, but I wanted to start walking him.  I bought him
several collars,  but  he goes limp and lifeless and lays down when I put a
collar/this applies to harnesses also.  I've tried everything to get him to walk
with a collar/harness on, but he refuses and lays down I lift his bottom and
tried coaxing with treats. I tried adding a leash, but don't want to pull on him
since he's so small.  This applies to in the house also.  He'll lay in an open
crate and not come out if his collar is on.   I've had to take his collar off to get
him to move to potty outside.  Can you help me?

Answer
Usually food works,and you don't want to drag him,because he is a small breed

Probably the best way to deal with this would be to call up a number of dog trainers and then then enroll the puppy in a dog obedience class with the trainer that you liked the best-tell them as much info as you can,the more they know the better they can help you