My son has already head feet and mouth. But I think he might have it again. Can he get it more than once?

Yes. There is more than on strain of virus that produces this pattern. Coxsackievirus a 16 is the most dominant strain but other enteroviruses (intestinal) are also known to produce these skin features.
Probably not. I assume you are talking about hand-foot-mouth, which is a virus causing blisters and sores on those locations, usually accompanied by a fever. Like any virus, once you've had it you theoretically cannot get it again, although repeat cases have been reported. I would suggest you see your doctor and find out for sure what is going on.