The majority of people have a decrease in blood pressure at night...(as well as a decline in heart rate). These folks are called "dippers". I believe that about 70% of the population reacts this way to sleep.

"Non-dippers" experience an increase in blood pressure during the night.
Blood pressure typically increases during sleep apnea.

So your son, Rachel may just be a "dipper" physiologically.

You can check this generally by taking a pulse at night, and compare it to
the 1 hr after rising pulse. It might just take your son that first minutes being upright to raise his blood pressure.

Dippers vs non-dippers are carefully tracked when hypertension is present.
Certain drugs taken at the wrong time per day, can lead to serious side effects in dippers. And non-dippers require certain care also in timing of their meds.

It is a well known fact in adults that the first minutes in the morning have the greatest heart attack and stroke statistics...this is because of the sudden shift in blood pressure and heart rate, as well as the other metabolic changes that accompany shifting from sleep to wakefulness.

If your son does NOT eat, and still pinks up about an hour after rising, then I would not suspect blood sugar. But you could still give him a snack the night before --a high protein one--to see if it helps.