Hi K.:
Sorry, I can't address the specific question of your son's on-going temperature regulation. However, hypothermia can quickly appear in small children from swimming pools in about 1 hour of time.
My husband was an avid scuba diver during his college days and is more trained about how the body loses heat in water, signs to look for. Before that, I wasn't aware that only a drop of 1-2 degrees in body temperature triggers hypothermia. (I had always thought of hypothermia as something with winter and frozen lakes...)
Below is some info I found on the web:
Hypothermia may develop within minutes of being exposed to cold water, or it may take several hours, depending on the water temperature. Water doesn't have to be icy cold to cause hypothermia. Your body loses heat more quickly in water than in air. Any water or surrounding temperature lower than your body temperature causes your body to lose at least some heat.
Very young age. Children usually lose heat faster than adults do. Children have a larger head-to-body ratio than adults do, making them more prone to heat loss through the head. Children may also ignore the cold because they're having too much fun to think about it. And they may not have the judgment to dress properly in cold weather or to get out of the cold when they feel cold. Infants may have a special problem with the cold because they have less efficient mechanisms for generating heat.
http://en.wikipedia.org/wiki/Hypothermia
Symptoms of hypothermia?
The following are the most common symptoms of hypothermia. However, each individual may experience symptoms differently.
Symptoms may include:
• confusion
• sleepiness
• fumbling hands
• shivering and exhaustion
• slow, slurred speech, or shallow breathing
• weak pulse and/or low blood pressure
• drowsiness
• a change in behavior or appearance during cold weather
• stiffness in the arms and legs
• poor control over body movements or slow reactions
• in infants, bright red, cold skin and/or very low energy
To determine if the person is suffering from hypothermia, take his/her temperature with a thermometer. If the person's temperature is below 96° F, call for emergency help immediately, according to the National Institutes of Health (NIH).
http://www.mayoclinic.com/health/hypothermia/DS00333/DSEC...
Normal body temperature in humans is 37°C. Hypothermia can be divided in three stages of severity:
In stage 1, body temperature drops by 1-2°C below normal temperature (35-38°C). Mild to strong shivering occurs.[1][2] The victim is unable to perform complex tasks with the hands; the hands become numb. Blood vessels in the outer extremities constrict, lessening heat loss to the outside air. Breathing becomes quick and shallow. Goose bumps form, raising body hair on end in an attempt to create an insulating layer of air around the body (which is of limited use in humans due to lack of sufficient hair, but useful in other species). Often, a person will experience a warm sensation, as if they have recovered, but they are in fact heading into Stage 2. Another test to see if the person is entering stage 2 is if they are unable to touch their thumb with their little finger; this is the first stage of muscles not working.