Hi, I'm Cher. Welcome to JustAnswer.
I'm sorry to hear that he may have licked rat poison in your neighbor's yard. If you're not sure if he licked it or possibly ate a pellet of poison, the best thing to do is have him seen in person and evaluated by the vet, plus have a basic blood test performed, so you know for sure how he's doing. They can give him an antidote, Vitamin K, to stop any internal bleeding, if this is necessary.
Rat poison contains a substance that stops the clotting of the blood and that's how it kills the rats or other animals that ingest it.
It's better to err on the side of caution and have him checked out in person, so you can be sure his blood count, kidneys and behavior is normal.
I would recommend just monitoring him at home, but in this case, the bleeding is internal and often, there are no symptoms until it's too late, so that's why I'm recommending a visit to his regular vet or an ER vet center, now.
I hope all will be well with your furry boy!
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