I am thinking that Chad is probably not a Dimeira – I am a couple days behind but unless something else has come to light, it seems to me that Stefano is pretty on top of this situation -like he knows about Kate’s connection to Madeline. So if he thought Chad was one of his offspring or somehow a grandchild wouldn’t he be all over him trying to bring him back to the family?
your line of thinking, however, the big question is, does Stefano have all the facts. True, he usually seems to know everything, but that may not have been the case when Chad was born.