If Zagreus can't help remind him, Mort will do his best. However, Mort can't convince others to treat Thanatos the same. They know him more as a treasured, lost Chthonic Companion not a friend who has been with him through all of this. As much as Mort will FITE for Thanatos, it would be a battle he couldn't win for Thanatos's sake. (And a battle Thanatos can't fight by himself, for himself, without major support.)

Thanatos doesn't reply. He just gives Ryo the hug, and he's gotten better at hugs than he once was. Mort hops over from Thanatos's shoulder to give a light paw pat to Ryo's cheek. "You will take care of yourself and... find your brother."

Maybe even give him hell for leaving. Reasons be damned.