My son has a small black spot inside mouth on check?

Black spot in mouth: Without more information, black spots in the mouth could represent a lot of potential problems. It could be a result of an accidental bite with a bruise. This is certainly not uncommon with kids. It could be a benign growth. It could even be something very concerning. Best to have it evaluated and if it isn't clear, he may need a biopsy to help figure it out. An examination is best start.

Black tongue: The spot can be many things... A nevus commonly known as a mole. Of less concern i would think would be black hairy tongue what we call lingua villosa nigra. Don't be alarmed this is a benign condition caused by bacteria or fungi in the mouth. Good brushing of the tongue should solve this problem. Finally it may be pigmentation spots which is normal. Some refer to this as hyperpigmentation.

Cheek bite: He probably bit his cheek. It is very common in the area of newly erupting teeth. See if it disappears in a few days. It can also be normal pigmentation - like a freckle. If it does not go away or changes character then be sure to mention it to his doctor or dentist. It is most probably nothing to worry about.