I would say he did not tell me because he never wanted Eric to be a demon and wanted him to die. Simply because he saw the boy as a disappointment and did not want to have a demonic son like him. And maybe even have him come and approach Eric and Ash (after they finish with the transformation) and try to mess things up. Or he could some how know Mabel and try to kill her and the boys. 

But he just did not tell him because he wanted to watch him die from afar, his father never wanted a son. Also another reason Eric and his father have no relationship.