If someone gets heat stroke and he doesn't receive medical attention, will he surely die?

High likelihood. Heat stroke is a medical emergency that has a high mortality if not treated promptly. Fluid replacement and electrolyte replacement are absolutely necessary.
Not necessarily. It depends on the severity of the heat stroke. There varying degrees of severity. Mortality is 5% if the person recieves treatment, but there can be lasting damage to brain or other organs, especially if treatment is not received when it is needed. The person should be assessed and treated emergently to maximize the chance of full recovery.