I am NOT a gastro expert, nor a doctor, but some things come to mind.
Perhaps the rapid dumping is just one thing that is happening, and not the only cause, just the only thing that's been found.
You mention every lab test we can think of - what about food allergies? Diabetes? Thyroid? For the thyroid, you need these tests:
Free T4, Free T3 and TSH to test actual thyroid function.
To test for Hashimoto's, you need antibody tests, Thyroid Peroxidase Antibodies (TPOab) and Thyroglobulin Antibodies (TgAb). You need both of the antibody tests because some have one or the other and some have them both.
What about Celiac Disease?
Is he on the opiates for his back? Are there other options? I don't know what his back issues are, or if a spinal block would be appropriate for it, or with his current problems, but ask for a referral to a pain specialist and see if there are other alternatives that may help. If there are, he will likely need to taper from the opiates - don't let anyone cut him off cold turkey. We have an excellent Addiction forum that can help with this process. (I'm not saying he's an addict, but they know all about withdrawal and tapering.) You can find that here if you ultimately need it - https://www.medhelp.org/forums/Addiction-Substance-Abuse/show/77
As soon as I saw you were in Southern NH, I thought Mass Gen. I'm glad you are going there.
Start collecting all his records - all his tests, all his ER visits, etc. Make a timeline - when it started, when it's gotten worse, all his appointments. Make a list of all his symptoms, even if they seem minor or insignificant. Note his activity (or inactivity). If it fills 3 notebooks, do it.
Start tracking his symptoms and food. See if anything makes it worse or better. There are a bunch of food tracker apps you can use. Many you can add notes to, so you can track symptoms after. Some allow you to print reports. Try a few and see if those work for you.
Keep us posted as to how he is, and what you find out. I hope this helps even a little.