Under normal circumstances, he would know right where to go. Hermione has shown him how to get into Diagon Alley, and he's positive there'd be some kind of magic herb shop or something that he could get some of this stuff. But if she's had to cut off her door, and he doesn't have any idea why, then trying to force his way into her universe could create more harm than it helps.

The otter's expectant look confuses him a little. It's clearly not Hermione herself, even though it spoke with her voice. A communication spell of some kind, clearly, but an otter? Makes no sense to him. "Does this thing work both ways?"