Ill admit, its a lot of maybes when you are dealing with life or death. Dexter has no idea how they might react if they find out hes assumed Reds identity under false pretenses. Prater is rich and has lots of resources. It wouldnt surprise me if he already likely knows Dexter isnt Red. I mean cmon, they broke into his home, saw his trophies, got his fingerprints etc. They really have no clue what he looks like? And Dexters face was on the news, in relation to one of the many serial killers Prater is obsessed with. Its not a stretch that Prater could easily make the connection and figure out hes Dexter Morgan. And at that point he will either think Dexter is working undercover for the police, or he might even be smart enough to connect more dots and figure out hes the real BHB.