There are different strands of hfm so you can get it more than once, poor ds actually got it twice. Much more mild the second time.

Our doctor couldn't tell initially it was hfm as we were too early for the classic blisters, it just looked like random little pin *****s at first.

I'd keep an eye on him and perhaps see the doctor in the morning to investigate possible ab reaction if it's in the family. Just to be safe.