In your original post, you said Moses was "low functioning", and since most parents over-estimate their kid's ability when they're autistic, I assumed that he wasn't able to communicate his wishes and didn't respond to stimulus appropriately. (let alone, be sociable!)
So is Moses able to tell you when something hurts and point to the pain? And if you would, please describe what kinds of communication that YOU ARE able to do with him, because so much more is possible when autistic kids can give you feedback. Also, will he/can he swallow capsules, and if not, will he take eye-dropper supplements mixed with fruit juice?
From the small amount I do know about your situation, it sounds VERY HOPEFUL that you've "taken the bull by the horns" and tried the diets over the years. And I would suggest that his loose stools are telling you that you're missing something, probably with gluten in it, and you may not realize how TINY AMOUNTS of gluten can render all your efforts worthless, without you even realizing. For example, most HAND SOAPS have gluten ingredients and even 1 CRUMB of a bread product may nullify months of faithful dieting, to someone who's highly immune compromised.
I know it sounds like "pie in the sky" after so many years of doctors and watching him grow up with impairment, but if Moses was a NORMAL child at birth, then regressed at the usual time (after the MMR vaccines) then you have an EXCELLENT chance of recovering him to the point where he can live his own life. (unless there's something else I don't know about his disability)
You mentioned chelating with DMSA. Was that under the direction of a doctor? (Incidentally, just FYI, DAN doctors are simply regular doctors who go to 1 day convention, and are pronounced DAN after the day. Some of the WORST doctors go to the DAN convention, because they don't have enough patients to fill their normal schedule, so they go, hoping to get a whole new clientele. I haven't been reading up on it for the past few years, but their initial "DAN protocols" were nothing short of laughable)
DMSA is an excellent chelator, but it DOES NOT PASS the BBB (blood brain barrier).
Autism is a toxic metal disease. And the mercury is stored in the brain, where DMSA can't get at it. (lipid tissue has a 5X preferential uptake of toxic metals over other bodily tissues)
It sounds like you have the resources to see doctors and such, so you should find a doctor who'll prescribe DMPS (dimercapto propane sulphate) which is the "cadillac" of mercury BBB chelation agents. DO NOT ALLOW a doctor to do "challenges" and IV administration of chelators, because while faster, they are also MUCH more dangerous to Moses and are completely unnecessary. Speed is something the DOCTOR needs, in order to see more patients. With more patience and less speed, you can administer chelation orally and avoid the horrible side effects of improper redistribution of the body burden, and actually get RID of the stuff, once and for all.
If you don't have the money for doctors, you can also use Alpha Lipoic acid, a strong sulphur-based antioxidant that is perfectly OK for BBB chelation. The downside of ALA is that it tends to create yeast problems, and most unexpected reactions from chelating are YEAST/CANDIDA related. (this includes repetitive behaviors, stimming, uncontrollable outbursts, spitting, you name it) Just be sure to get the ALA from a store or online source that sells a lot of it, so that you can be sure it's fresh.
I should pause here, and tell you that I speak from PERSONAL experience, as well as with our experience with our grandkids and hundreds of others we have known. I was probably the MOST MERCURY TOXIC person you'll ever meet (that lived to tell about it) and I am INTIMATELY acquainted with all the problems of acute toxic metal chelation.
The CHELATION of Mose's brain burden of mercury, is the SINGLE MOST IMPORTANT THING YOU CAN DO. This will take AT LEAST a couple of YEARS, but will be well worth it. (based on my experience)
You don't need a lot of tests, or doctors, or anything else that costs a lot of $$$. All you need is a bottle of chelator, some critical antioxidant supplements and support vitamins, and the faith that will see you through the scary times of "what do I do now?"
If Moses was of normal intellect before his regression, it is very likely that he's much smarter than you think, and once he's talking, he'll probably tell you that he remembers all those years that you took care of him, but he couldn't put together the sounds in his head in order to talk. (this has happened time and again, with kids who were once diagnosed as "institutional" candidates)
Let me know if you plan to try the above suggestions. There's much more to it, of course, but I have to get to work and I need to know more about your situation as outlined above.