Given his attitude, he may feel no obligation to accept, of course
Just as Cedwyn would never accept a challenge from a peasant, the Decimus may consider Cedwyn beneath him and just have his guards beat you for insolence.
It's a risky proposition, but none of your stats are above 16 so you're free to determine your own action.
Now, if the guy called Arne's Honour passion into account (24), Arne would have a very hard time refusing that challenge.