My fourteenth week old son is constantly gumming on his hands. Is he teething early?

Possibly, He could be teething the lower incisors. Or, he may be just suckling if he doesn't use a pacifier. No real harm in this.
One Possibility . He may be teething or just trying to find his thumb or finger. Try messaging his gums. See if you can feel a tooth ready to emerge. It will probably be in the front on the lower jaw. He may like it if you brush his gums with a soft bristle brush. If you can guide him to take a pacifier rather than a thumb or finger it will be better for his bite development. Ibuprofen usually works best for pain.
Possibly. Normally the first eruption of teeth occurs at six months but some happen later and some children are actually born with teeth. The constant gumming of his hands is a definite sign of teething.