@SOC I've just started reading your "How long does your PEM last?" It is very enlightening. It looks like it is going to take a long time to figure all of this out.
Ain't that the truth! Sigh.... People who have been dealing with this illness for decades are still trying to figure out how to manage it.
Your observation that, "More energy does not directly correlate to less PEM." sure rings true in our house today. He was trying so hard to mostly recline in the zero gravity chair yesterday(it help his POTS), when he really felt like sprinting through the yard. Just a simple car ride...I don't know if the POTS or the exertion triggered it.
POTS seems likely, but obviously nobody knows for certain. Is he being treated for POTS (or some form of OI)? Is he fluid-loading at night and in the morning? Is he drinking 2-3 liters of water daily, at least 1 liter of which is electrolyte water? Does he wear compression socks? I doubt you'll make much progress with PEM if POTS is not under control. In fact, it might not be PEM, but exercise intolerance from dysautonomia which is a difference beast and more amenable to treatment. If that was under control, you might find his PEM threshold is a lot higher than it currently appears.
Did he do more than ride in the car? Walk around in a store? Walk more than usual going to and from the car? Was there a lot more noise or visual stimuli than he's used to? The exertion of a car ride could PEM him, especially if he's mostly bedbound, but POTS seems the more likely possibility if he doesn't ordinarily sit upright with his feet down. There were years that I couldn't sit at the dinner table without difficulty. I was either in bed or a recliner with my feet up, including for meals.
He has a 2 hour car trip to see Dr. Chia coming up on Thursday, it's scary to think what might happen.
Any advice on how to minimize POTS in the car? Seat belts can't do their job if one is reclining.
When daughter and I were first seeing our original ME/CFS doctor, we couldn't sit up in the car for the whole trip -- 6-7 hours. We took turns lying down in the back. We got a couple of seatbelt extenders so we could lie down and belt in with two sets of belts. If he's a small guy, you might not even need the seatbelt extenders, just loosen the seatbelts as far as they'll go. Admittedly, it's not as good as sitting up and being belted in normally, but it's pretty good and probably sufficient. It got us to the ME/CFS specialist, which was critical to improving our health, so it was worth it.
Beyond lying down in the car, you can make sure he's well-hydrated by fluid-loading (500-750 mls electrolyte water before bed and again before getting out of bed in the morning) and making sure he drinks a lot of water all day. There are some ways of checking whether he's dehydrated. (Don't take my word for these; look them up to make sure.

) One is narrow pulse pressure -- the difference between systolic and diastolic BP is 30 or less. Closer to 40 is best for a lot of us. Another test is the pinch test for dehydration (look it up).
You could also try to find ways to get his feet up during the car ride. Perhaps put him in the back seat, and fold the front seat down so he can put his feet up on it. Again, not as safe as sitting upright normally, but not a huge additional safety risk.
He tried the CoQ10 before and it made him feel worse. Now I'm wondering if some approaches can only work if certain other symptoms are being treated? Maybe some treatments require supportive synergies.
I think Broderick's work is suggesting that -- that certain treatments work better if done in a particular order.
CoQ10 affects different people differently. Some people feel very wired on it. I suspect response to COQ10 may signify degree of mitochondrial dysfunction, or some type of mitochondrial dysfunction, so those who get wired on CoQ10 have enough CoQ10 already and adding more just over-revs them.
In my experience, it's almost impossible to tell if some treatments are helping if OI/POTS is not under control because one is constantly having POTS episodes which confuses the issue.
Does Dr Chia treat POTS? Have you talked to him about Florinef or other drug treatments for OI? Do you know if your son has POTS or some other form of OI?