I would definitely trust a vet before my own knowledge, as you should, but this SOUNDS like just too much stuff too soon on an already compromised body/system. Trying to combat and minimize ALL signs/symptoms can get overdone quickly in a weakened body and eventually it can (doesn’t mean WILL) all back up on him (colic, systemic shock, absorption difficulties, laminitis, abscesses, etcetcetc). Consider going back to the most basic feed programme (limited sugars, NO corn, moderate protein, moderate carbs/ starch and free choice hay) and let him settle. I would add only SmartDigest for now, maybe an antacid, and PLENTY of plain ol’ warm water in his grain, his water bucket, any way you can offer it. Until he puts some weight on, SLOWLY and is showing more energy and interest in his options, I would be inclined to keep his diet simple with multiple small meals thru-out the day (Senior feeds are usually pretty nutrient-dense even tho easier to digest so small doses more often might be a little easier for him to take on till he RECONDITIONS his gut).
Dietary results can take up to three to six weeks to be seen in a normally working system (dealing with a large and intricate, highly sensitive equine digestive system here) so make patience and time your new best friends for now but watch him like a hawk. Learn his tells of pre-struggle.
He could probably use deworming but a fecal test would be the best way to go there, treating him for what he actually HAS as opposed to dropping a broad-spectrum chemical load into an already sensitive system. Might even want to wait on that but again, this is a vet judgement. Bet his teeth could use a float, too. All these little things put together contribute to good digestion and use of foodstuffs.
Hand walking and spending time grooming and loving on him, too, will encourage interest and security, security being a critical point in health, imho, in his new life and person : ) The average horse body at 1000+ lbs is operated by a 2 lb brain, stroke the brain and the body tends to follow. Make him feel good in his head and his body will settle, too. Good luck with this, and on behalf of ALL the horses out there that deserve a better life, thank you for saving this boy, he is lucky to have you : )
Just to be clear, I am NOT advising that you bypass your vet’s recommendations, just offering food for thought, no pun intended.
Sure there's right and wrong but mostly there's just a whole lotta different.