Wait for him to go away, which he does occasionally; for example, try messing with the flies, and he takes off.
How do I fix the flies?
Then just open it. Nothing there but an unmarked script --
-- until you get into the "Personal Notes" section of Grigor's tablet, that is.
How do I get into Grigor's tablet?
The note "What's in a name?" says something about ripping the red stitching in Grigor's satchel. Ha! Once Grigor leaves again (which he does once you finish with the flies) --
-- you can go to the bag and just rip open that stitching; no need to look for a knife.
This is ... odd information. You probably ought to ask first the Hardy boys and then Grigor about this.