Ah, that lovely virus. Funny you should ask, as I have blisters on my fingers as I type. It's been spreading around my area this summer & I seem to be oddly susceptible.
The virus hit us quite hard 2 years ago, then last year was a milder version of it, then this year just a few blisters w/some extra sleepiness (and crankiness on DD's part). It's odd that I have been affected along w/DD because everything I read suggests adults do not get it, or if they do, it's very mild.
Odd thing about this virus is the diversity of symptoms. Some kids are merely cranky and otherwise asymptomatic. I noticed the incubation period to be about 10 days after exposure.
Full-blown coxsackie at our house looked like this: incredibly clingy and cranky 2 yr old, did not want to walk anywhere. Subsequently (about 2 days later) noticed blisters all over the bottoms of her feet. (When I started getting the blisters I realized why she didn't want to walk - they were seriously painful!) Over a period of hours she broke out in blisters all over her fingers and mouth. She refused to eat. Terrible headache and very tired. A low-grade fever, around 101. (I had all these symptoms as well.) The worst part was the blisters, which for us persisted for about a week and really, really hurt.
This is the worst case scenario - as I said, I know lots of kids (in fact, the one whom I believe infected DD) who were out of sorts for a day, and then 3 days later exhibited one lone blister. Hope that helps, and hope your little guy does not get it!