My son's adult front tooth is very pointy. why is that?

Varies.: This is something that is hard to determine without knowing more information. Your sons age, developmental history, eruption of other teeth, trauma .... Etc.

Could be microdont: If it is fully erupted and does not appear to have decay or fracture, but is small and pointed it is possible that it is a microdont (small tooth). If so it would only need cosmetic work to fix. It may be an extra tooth as well. See a dentist for treatment options.

Need to be seen: Hard to say from the info you gave. Could be peg lateral. Take him to the dentist.