You tried to talk to him? That's bad. Some help wouldn't hurt.
Duncan is not responding
Did you spot the telephone at the lower area?
What about the telephone?
If you use the telephone Cuddles will call to Tabby.
I reached Tabby
If you talk about Duncan to Tabby she will give you some hints.
Duncan likes milk and Cuddles will probably come up with a ploy for Duncan.
What next?
Go talk to Duncan, again. As you have talked with Tabby about Duncan, Cuddles will fool Duncan that there's a milk tycoon outside. Duncan will wake up, momentarily.
Duncan gets back to sleep...
Maybe there should be a cat outside so that Duncan believes you.