A great place to post about him or ask if anyone has reported him missing are tack and feed stores, as most local horse people will go there to get supplies. If he truly only has four teeth then he is very young (4 to 6 weeks old) and will need additional supplement feeds to make sure he grows right -more than just grass and water. Posting at least what country you live in and region might help us suggest more things to help you out. As others have recommended I would suggest you talk to other horse owners in your area and see if they can help you out to learn what you need to to take care of this foal. He really needs his Mom still and to be raised with others of his kind to learn how to behave.
The fact that he is so friendly indicates that his early life was filled with human interaction, otherwise he would be more scared of you. I am sure that there is someone out there missing him.