3 week old temperature

My sons pediatrician told me if his rectal temperature was 100.4 or higher to take him to the ER. Last night I took his rectal temp for the first time and it came out to be around 98, but when I took his forehead temperature it was over 100! I don’t know if I should be concerned since his rectal temp isn’t high. I tried taking his clothes off with just a diaper and put him in a cold room to see if that will help, but he still feels hot to the touch. I need advice!