My 7 yo son has a patch on his tongue that is a darker shade than the rest. Looks like someone cut a layer off. Rest is whiteish. No pain. Any ideas?

Geographic tongue. There are some that demonstrate a process called the geographic tongue. This is more descriptive as sometimes it can look like continents (gray) surrounded by sea (pink-red). The tongue is populated by thick cells that accumulate & turn grayish.They shed periodically, replaced by newer pink-red cells. If older cells separate in a big patch, this appearance is seen. It is benign.
Geographic tongue? Your son probably has what is called geographic tongue. It is a migratory area of inflammation that causes the follicles of the tongue to swell. When they are swollen it looks more glossy compared to the surrounding areas, commonly there is a white line surrounding. These areas can move to different locations on the tongue surface, looking like a "map", hence the name g.T. He should have an exam.