First! FRESH, CLEAN WATER, ALL DAY, EVERY DAY!
Ok, don't fret, dear, at all the seemingly harsh comments. My first horse ever was a rescue, about in the same shape as your young male. Find out if he still is a stallion or has been neutered, and if not, that's the first thing you need to do. It will help him regulate himself and gain the weight as opposed to run the weight off looking for mares.
Just a suggestion, to get started, worm them each for 100lbs over their weight. Use either Equimax or Zimecterin Gold, which you can find at Orschelns or Tractor Supply. They are the more expensive of the wormers, but they are the best for first-time wormings. In 7 days, give the same dosage of pyrantel pamoate, or ivermectin. 21 days after the PP or Ivermec, give the same dosage of whichever those two you didn't use the first time. Keep them on that rotation, worming once every three months.
Ill horses should be treated slowly, and once you get them home you CANNOT just dump feed to them. This can cause colic or other serious digestive problems. Considering the horses conditions, they will need a gradual ration. They obviously haven't been fed very well, and the enzymes in their stomachs aren't conditioned to digest so many different things at once, so throwing them out to green, grassy pasture will do them no good. Make sure they have hay ALL DAY to eat at will. This will get their stomachs on their way to normalcy.
Start with a plain prairie hay, or prairie/brome. Timothy, clover, and alfalfa are higher protein, and these horses bodies cannot handle that stuff at the moment, nor do they need it.
Here's how I tend to start mine off:
I feed Platform feeds, by the way.
In a huge bin, probably 40 or 50 gal tupperware tub (with a lid! keep that stuff covered to prevent rodents and flies from contaminating it), I mix:
one 50lb bag of senior pellets,
one 50lb bag of mare/foal feed,
one 5gal bucket of Omegatin pellets (a fat supplement).
I mix these up really really well, and start with half a coffee can once a day for the first week. The second week, give half a coffee can twice a day. Week three and four, 3/4 coffee can twice a day, week 5 you're in the clear to provide a full coffee can twice a day.
On week 5, for your little stud, I would get a small tub of "GrowColt" supplement, and add one little scoop once a day in the feed. (provided in the bucket for you) Week 6, one scoop twice a day in the feed.
This will take a LOOOOOOONG time, I bought my only mare last June at 658lbs. We weighed her again June this year, and she is only up to 890. She's not as big as she needs to be, but is doing very well, as you can see from the pictures on my profile.
And ladies that are afraid the mare is pregnant, there's no way to tell by looking, the vet will have to check. Looking at my mare you'd assume so too, but she's just potbellied, and hasn't even come in heat yet this year...
If you have any other questions for me, feel free to PM me!
Good luck with your babies!