My 2.7year old son has a bad cold, running nose (mucus) and cough. no fever. unable to sleep properly since 3-4days now. what medicine to give?

Not much: For a child this young, I would recommend against any prescription cough/cold medications and even over the counter cough/cold meds as these can have significant side effects and are not shown to be effective. Simple home remedies like warm water with honey and lemon before bedtime, breathing in steam in a hot/steamy bathroom before bedtime, and Tylenol (acetaminophen) or ibuprofen for any pain/fever relief.