very tiny three week old (please help)

A stray had kittens in our garage, I always heard stray kittens are better off being left alone with mama so I limited myself to handling them. Right after they started opening their eyes, one died, and when I removed the deceased one I realized the orange one (we call him orangey because why not) was BONE thing, and acting lethargic. So we rushed to the store and bought kmr and starting feeding him, so here we are 2 weeks later, and two remaining kittens are still in the garage, she moved them from the box on the shelf to behind some old mirrors and cardboard on the floor and they started walking a bit. The tabby cat is a normal, 3 week old and is a pretty big boy, but orangey on the other hand isn't so big..He is 3 weeks old and still weighs 4 ounces. We feed him plently of KMR and he doesn't act weak or anything like that, he's just still very tiny. He walks a bit and doesn't stumble, and isn't as boney as he was when we began feeding him. He was biting this morning while feeding him and I can feel him teething. Does anybody know why it's taking him so long to grow? He is basically a 3 week old in a 1-2 week old body. He isn't starving or anything, he is just not growing fast at all. Do you think it has to do with him being so close to death that it's just taking him a long time to get back on track?

Thank you for reading! And hopefully I can get an explanation

You've done great with him so far. He is very,very small. He may be so because he lacked vital nutrients when he really needed it. Can you keep him inside, away from the others, and feed him exclusively? I am afraid being this small, its somewhat of a miracle he's made it this far. Also you will need to keep their mother from taking them outside unless the door is locked, else they could get lost or become feral as they are very young. It will be good to socialize them now rather than later, so they can get good homes. I am tagging @catwoman707   and @Feralvr   they may have better advice. By any chance could you call a Vet and see if they might have any kitten vitamins you could have for him?

Well he certainly is tiny.

He weighs what the average kitten weighs as a newborn.........so this is not sounding so good.

I do have to say though, that sometimes a kitten is just behind, but 4 ozs is VERY behind.

He needs to start getting weighed daily, with a grams/ounce scale, so you can feed according to him gaining the appropriate amt of weight each day.

It should be 8-10 grams gained daily, bare minimum is 6 grams.

Do you monitor how much he eats a day? Can you post pics of him?