J.S.
Doesn't it concern you that the preschool allows him to go to school like this? Wonder where he got it from? Things that make you go hmmmm!
Anyways... since it's an antiviral they do have antiviral meds that the ped could have given him. However, let me tell you some natural sources that are also shown to be anti-viral. You will need to do your research since I dont know much about HFM Disease. Garlic and St. John's Wort are both antiviral in nature. (Garlic is also antibacterial) You can do liquids and make an smoothie for him to drink which would be soothing and cooling to the sores. Stick with things like bananas and apples and pears as opposed to berries which are more acidic.
Also, get him on some probiotics which strengthens the immune system.