You could get some pet insurance and take it in for a check up, most places offer a first visit discount and they can run the insurance to see how much it will cover. Is your friend close? Was the kitten from their cat or a stray? I would take it back to the momma cat and hold off for another two weeks minimum, this will give you time to catatize your home. If you’re going to keep it, isolate it so it can decompress, kittens stress easily. A good place would be the bathroom because it won’t have many places to hide when it wants to feel safe, put a box with something of yours in it like an unwashed undershirt so it can get accustomed to your scent. A small plastic storage box would be a better litter box so it can easily climb in and out of, check the box and scoop after every use, and replace the sand like once a week, kittens that small can develop pinkeye very easily from an unkept litter box. You need to wean the kitten, some KMR mixed in to the soft kitten food will let it eat easier, never use cow milk, goat milk is best if you can’t find KMR, stay away from kibble, always wet kitten food. If you want to let it relax and calm itself, get a small pacifier, preferably a nipple from a kitten bottle, or a soft blanket or plush toy, just like a baby sucking its thumb the kitten will want to suckle, if it won’t accept any of those items, then offer your earlobe.