Time to put some practical connections to the rest of his everyday life with the carrying. Pretty soon, perhaps even before he goes out tonight, he is going to have to use his rope bone as a "ticket" of sorts to get to do things.
Hopefully, this will make the connection even stronger and he can see that this isn't just a behavior he needs once in a while, but something he can always do. It would go towards the generalization process with the behavior as well, as the situation will always change.
I should also introduce it into our shaping games, so he can offer picking it up and holding it more often, another way to further entrench the behavior into his mindset.
Hopefully, this will help him also be more likely to carry other objects. I know he can physically do it, it's just a matter of getting him to overcome the fact the object might not be familiar or have his preferred texture or be a certain size or weight (assuming it's not too heavy to carry of course).