I am very happy to hear the runt is thriving! For future reference, I CANNOT stress enough how important it is to let the mom raise her baby bunnies. There are obviously exceptions, but if you feel a doe is rejecting her kit, at least help it nurse off her 2 times per day. A kit NEEDS it's mother's milk in order to grow / develop properly. KMR should only be used in a pinch (such as if the mother dies), but it lacks essential rabbit nutrients and vitamins.
To help a kit gain weight, at 3 - 4+ weeks old, give them a teaspoon or so of oats and calf manna daily to provide additional fats and calories. Keep an eye on what goes in (and out) of their body. Critical care can be fed to older kits to keep them alive. Syringe feeding water can help keep them from becoming dehydrated. If possible, leave them in with their mom a few weeks longer than the other kits are weaned to build strength and reduce stress. But remember... no matter how hard it seems, it is always best to let nature take it's course. Maybe the doe was rejecting this kit because she knew it had a slim chance of survival. There is sometimes an underlying health issue that may make the rabbit suffer. It seems like you got lucky, though! Thank you for the update ♥