Tough to say. I really don't know anything about the actual history which I understand is only partially accurate anyway.
They already had him betray his brother once then reconsider. Would they do it again? Maybe he accepts the offer but with no intention of betraying Ragnar but instead betrays the French king when the vikings return.
Yea its tough to say. I'm hoping he ends up betraying the French, but I wouldn't put it past him to betray Ragnar again. His moment alone with Ragnar's casket made him seem remorseful of his wrongdoings in the past, but yet accepted it like he couldn't change himself.