My Dear The cold and fever that your husband got is most likely viral condition It is well known to experience rashes on the upper trunk especially chest and shoulders accompanying viral diseases due to viruses entering the blood circulation and modifying the components of immune system to result in such rashes. there is nothing to worry as such rashes tend to disappaer by themselves but sometimes with slight scaling or flakiness. You might apply some moisturizer over the rash and give him an anti allergic which will help reduce the rash faster beside providing relief from the itch. The rash is not connected to Vitamin C that he took. Please do not worry, He will be alright.