if your baby has a cold, when do you take them to the doctor?

my sons cold started 3-4 days ago. his cough is getting worse, but it isnt persistent. its starting to sound a little wet and maybe even phlegmy. but I haven't seen any phlegm.

I don't have medicaid, so I would hate to take him to the doctor and they cant even do anything for him.

when do you take a baby? what do you do for the cough?