We had HF&M in February and it *sucked*!!! My 5 year old was hardly affected: just a few spots on her butt after a day or two of the fever. My 3 year old was wiped out for about 4 days and had a few blisters on her fingers, toes and in her mouth. My 11 month old and *I* had it the worst. She was COVERED in red, raised bumps - I was sure it was chicken pox! But they didn't seem to bother her. 3 months later and there's still the "shadow" of them under the first few layers of skin (it is almost gone though). I had them on the soles of my feet and palms of my hands and they itched SO SO bad!! The worst of it only lasted about 2 days, and then it subsided.
Your son may get a few more, but once the blisters start to appears it usually stops spreading within 2 days or so.
Oh, and don't freak out when the skin under the blisters starts peeling off - it's painless, though kinda gross. And also, when his nails start flaking off. It's from the after effects of the blisters. Again, not painful unless a piece of attached nail gets caught and rips off. We're just about done with the flaky nails (3 months later!) and the new nails underneath grow back just fine on their own. I've just been gently clipping off the extra nail to keep them as short as possible and we haven't had any problems.