Some kids just can't stay consistently dry until well into their elementary-school years. Even though your son had some early success, he may be experiencing heavier sleep cycles now, or dreams in which he actually goes to the bathroom (I occasionally have dreams like that) but, of course, he's still sleeping.
He cries and acts ashamed, so he obviously isn't doing this on purpose. I would just tell him that his body isn't quite ready to stay dry at night, and let him wear diapers or pull-ups. Let him know you're confident that he'll be able to stay dry some day, and aren't worried about it.