NAD but have been through the POTS stuff for 6 years. Take what I say with a grain of salt (heh):
Has he had a tilt table or a orthostatic tolerance test? At least regarding POTS, adrenaline surges can cause a lot of the issues you've mentioned, and those would be good places to start.
Speaking of adrenaline: you've mentioned two other 24-hr urine tests-- has he had 24-hr adrenaline done?
Some of the gastro stuff could be connected to POTS- just the act of digestion can cause exacerbation in symptoms due to the increase in blood flow to the gut. Intestinal motility issues (constipation) is also pretty common (according to my neuro).
I really hope you get the answers you need! In the meantime, grab some compression stockings, and encourage him to alternate between sports drinks and water. I know it sounds like a lot, but it really is the best option. I also aim for around 5-6 grams of salt a day- start keeping track. I usually start my day with a "tea" of broth and 2 large glasses of water, drunk quickly. Best of luck!