While I can appreciate your having developed a bond with your little "mocker", I firmly believe wild animals that are rescued should in most cases be released when they are able to fend for themselves. There are
wildlife rescue and rehbilitation organizations that can better care for wild animals and properly safely re-release them into the wild or care for them until they die a natural death.
That said, if you keep the bird be aware its diet will change as it grows up. Mockingbirds have a very varied diet, eating everything from beetles and worms to berries and fruits.
I am not aware of the laws regarding keeping mockingbirds as captive pets, but my greater concern is that it's cared for properly.
As a warning to others, it's truly best to leave wildlife alone even when you suspect it is injured or sick. Call a local wildlife rehibilitation specialist or conservation group, rather than trying to improvise an animal rescue yourself. In the case of baby birds, you can not always be sure the adult birds have "abandoned" the nest. I have found several baby birds over the years (all mockers, in fact) that have fallen from their nests near my house. Even in the instances where I never saw the "mother bird" return to the nest or fallen baby, the babies have fledged and flown off -- I later see them on our fence and feeding from a backyard bird feeder, unscathed. Those that perish are, quite honestly, part of the natural cycle of life...it is, indeed, why many birds lay more than one egg or breed often.