Dehydration. Does it contribute to a senior's fall risk?

Yes to everything Eyerishlass wrote. They're all contributing factors. Dehydration can easily weaken an elderly person.

And the "liquid" he drinks shouldn't include pop.

Yes, dehydration can contribute to a fall. It can cause low blood pressure which can cause dizziness which can cause a person to fall.

You said he's drinking 5 or 6 tumblers of liquid a day? What kind of liquid? Water? Juice? Vodka? ;-)

Encourage him to drink more water or juice. Sometimes using a straw makes it easier.

Since your relative fell after getting up from a nap it could be that his blood pressure fell a bit when he went from a sitting to standing position. This is called orthostatic hypotension. Next time he wakes up have him sit at the edge of the bed for a minute before standing up.