I have a 4 month old son and he's been stiffed up ever since he was born my doctor told me to use nose stray but that didn't help now he has a cold and I don't know what to use

You take him back to the doctor. Do NOT trust the health of your newborn to strangers on the internet.

While I am certified in treatment of babies this young, it behooves me to give you any ideas of any treatments since I do not know the health history of your baby or any medical background for that matter.