Let's look at this at a different perspective. You have a child living with you. He wakes up out of the coma and spews things about not being related. Naturally, that means the kid is having some kind of mental illness. Especially if he acts out or suddenly moves away. You don't understand what's going on. It's like watching your parents have dementia. You can't just abandon them, but you also can't just get close to them either. It's that kind of thing. 

 

Also, you're in a different body and they could do a DNA test proving you're related. Which means more proof that you're having some kind of episode or something?

 

Getting a new identity is also a tricky one because it's insanely difficult to get a good quality ID. It could easily be detected in the airport, or other places if he wants to move from place to place. You also need money to go, and most likely if your credit card is blocked, you're basically stuck in one place. 

 

So it's much, much easier and convenient to just play along. Also, you don't slap a smiling face. Which is why it's easier and convenient. 

 

…I have a full list but… I'm worried I'd overwhelm you a bit here.