Eczema?

Hi everyone, so I know it’ll probably be asked or suggested but yes I took baby to doctor and he stated that it’s hard to tell where it’s coming from and if it’s viral it’ll go away on it’s on. It seems as if now baby has developed some patches on his neck and armpits. I’m not sure if it’s eczema or just a rash? I know I was told it’ll go away over time but it just worries me everytime I see it so I just want to know if anyone has experienced the same thing. He has no fever and isn’t sick or in distress. Not sure what to do or if I should go back to the doctor :/