My son, who is now 15, had HFMD when he was around the same age as your son. He had it so severe that the blisters nearly covered his teeth. I had never heard of it and like you, was a bit afraid until I found out what it was and what caused it. Anyway, to help you out better, I went to ask.com and found the info I'm sending you on the CDC website. The following I copied and pasted from that website. Hope this helps answer your questions.
Infection is spread from person to person by direct contact with infectious virus. Infectious virus is found in the nose and throat secretions, saliva, blister fluid, and stool of infected persons. The virus is most often spread by persons with unwashed, virus-contaminated hands and by contact with virus-contaminated surfaces. Infected persons are most contagious during the first week of the illness. The viruses that cause HFMD can remain in the body for weeks after a patient's symptoms have gone away. This means that the infected person can still pass the infection to other people even though he/she appears well. Also, some persons who are infected and excreting the virus, including most adults, may have no symptoms.
Washing hands frequently and correctly and especially after changing diapers and after using the toilet.
Cleaning dirty surfaces and soiled items, including toys, first with soap and water and then disinfecting them by cleansing with a solution of chlorine bleach (made by adding 1 tablespoon of bleach to 4 cups of water)
Avoiding close contact (kissing, hugging, sharing eating utensils or cups, etc.) with persons with HFMD.
Again, I hope this info helps. Honestly it was easier than typing it myself. :-) If you need more help, just go to the cdc website or type in your search engine....hand, foot and mouth disease and it will pull up different web pages.
Hope he is feeling better. God bless.