My child stepped on something a few months ago and now there's a black spot on his foot that's been there for about a month. What could it be? Is

Lots of choices. It could be anything from having a piece of whatever he stepped on in there, to a wart, to a 'tattoo' of dark pigment under the skin. Please have his doctor take a look and see if anything needs to be done for it.
Anything. Best to have it checked out. It could be a splinter or a foreign body (this usually hurts though) or a hyperpigmentation.Had a patient with same scenario, his was just dirt.