Can my son take fever reducing tylenol after taking nyquil(cold and runny nose only)?

Check the list of ingredients VERY CAREFULLY! Many Nyquil formulas already have acetaminophen in them already. If you are absolutely sure that it doesnt contain acetaminophen then you can go ahead and give him some Tylenol (acetaminophen) The bottle should have the ingredients listed but if you are unsure, call your pharmacist. Giving a child a double dose of acetaminophen can be very dangerous to his liver and kidneys. Acetaminophen is very toxic in large amounts especially to little ones. Always be sure when you are giving children medicines, especially liquid ones, that you use the proper measuring device and carefully draw up the correct amount and always be sure that when you give more than one medicine that you be sure you are not giving something that has the same substance in both formulas.

See also:

https://www.drugs.com/cg/fever-in-children.html
https://www.drugs.com/cg/acetaminophen-overdose.html