See doctor. I disagree that this is herpes, which rarely causes sores inside the mouth. Viruses can cause "canker sores", and there is a specific virus called "hand/foot/mouth", which also causes blisters on the hands and feet. Most mouth ulcers heal without treatment, but if your son is in pain and/or is having trouble eating, see your doctor.
Need more info. Your son should have a follow up exam, could be many things canker sore, strep, thrush.
Depends. Was your son at the doctor when he had his fever? It is possible that he picked up one of the herpes viruses that cause cold sores. It is also not uncommon for a child to develop rashes in reaction to a virus. However, there are more serious causes of these ulcers, and you should visit with your son's physician.
See your physician. The most likely virus causing sore would be herpes simplex, which appear when the immune system is weak. There are several medications that may help such at abreva which is otc. Sometimes the combination of an oral antiviral and a topic cream will reduce the time that the sore is present. See your physician to make sure there is no underlying illness such as a bacterial infection.