You are definitely not the only sucker on this planet. If you go to this link you will see my latest project. The last picture of him was taken late summer/early fall of last year. Rescue, before and after.
Like yours foundation bred QH with no papers because the owner hadn't bothered to keep updated records but I did see his dam's papers and the breeders certificate for his sire, 2 year old horse colt. He had never even had a halter on let alone been wormed, feet trimmed or anything else. I wasn't sure he'd even live because he was nothing but a bag of bones that was covered in burrs and lice.
I know many people disagree with the way I feed because I'm still old school when it comes to that and as you can see it worked for him. I have my feed mixed at a feed mill.
300 lbs of cracked corn
200 lbs of whole oats
100 lbs of horse pellets (don't know the exact brand name but every feed mill always knows what I'm talking about)
5 lbs of trace mineral salt
And then I tell them just enough molasses to cut the dust
I got that recipe from a long time breeder that I bought a couple of my horses from close to 30 years ago. I have no issues with colic or it making them hot. I do need to add that along about May I switch to just plain oats because with our humidity the feed will get moldy.
For any horse that needs to put on extra weight I use calf-mana for an additional supplement.
For the colts rehab he got 3 meals a day and very limited grass. Starting off he got about a pound of feed, 1/4 cup of calf-mana, a good size flake of second cutting orchard grass hay, a half a flake of alfalfa in the a.m. At noon it was another flake of the grass hay and another pound of feed (no calf-mana added), his evening meal was identical to the a.m. Feeding. I slowly built up from there, very slowly. Of course fresh waster, plain salt block & tm salt block was available at all times.
Once he got gelded which was the day after that "6 weeks later" picture was taken I started getting him used to pasture. Once he could be on 24 hour turnout he only got grain + a flake of alfalfa/grass mix hay once a day. Keep in mind by he time he was on 24 hour turnout it was probably late July early Aug. So the grass was not lush.
Today I classify him as an easy keeper. I still grain him 1X per day because I want them all (12 horses) to come to the barn so I can check them over, but he only gets about 1/4 of a scoop, if that much.
He's been an awesome little guy who has never given us one reason to regret bringing him home with us and I hope you find as much fulfillment with your colt as I have with mine. And bless you for being willing to try!