I wouldn't give the little guy any grain, just milk or replacer, and a good quality grass hay. They are finding a correlation between high protein feeds and urinary calculi in wethers. Your doeling doesn't *really* need grain either, unless she's in late pregnancy or lactating. A good quality hay is really all you need.
We sell most of our wethers for meat, so we do grain them until they are sold. They never get old enough to have problems from being banded young or grained. Just a difference of end use.
I have heard horror stories about using replacer-but we have used it many times, and I have never lost a kid on it.
There are many things that can upset a kid's digestive system, like eating too much or too fast, or being switched suddenly from momma's milk to replacer (or to cow's milk). All changes need to be done gradually. I hope the breeder sent you home with milk to transition with, whether you choose replacer or cow's milk.
For our lactating does, we use a 17% goat feed mixed with 9% sweet feed. Contrary to what people may have you believe, higher protein is not always better. Goats are not meant to be processing that much protein. Free choice WATER is the most important, followed by a good quality hay.
We also do not use medicated feed, never have, and don't plan on using it in the future. We don't vaccinate for overeating disease either. But that's a whole 'nother bag o' worms.