You should choose random parts, and add "I don't think this makes sense, can you explain this part?" or "Why didn't you use instead?", "I think you should have done " here." to random parts of the PR. No need for it to be true or read it, not like it matters. You can keep doing that for quite a long time :D Sure, they'll just copy-paste it to their agent, but the longer you keep that going, the more unsure will they be, probably.

Or even better, just message him that you have questions about the PR, if they can come over because it's more things, and ask these comments directly.