Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
Him trying to get her to change her will is called "undue influence" if he is a PoA
and he is acting not for her best interest but self dealing for his own interests. However, you need to protect your grandmother here and even if it may start a war with your uncle, you owe a duty to her to protect her as you have been doing all along. To protect her, at this point since your uncle is persisting in this type of conduct, you really need to hire a local attorney and you need to seek to get a court ordered guardianship in place to remove any of your uncle's authority over her, because as PoA he can try to change the will himself and that would cost you a lot more money in legal fees later than it will now by getting a lawyer and getting yourself appointed guardian over your grandmother.
Once you are guardian, you have rights to all the documents and will and information on any assets currently owned by your grandmother, so you can properly manage them.
This is the best way for you to protect her.
The only other thing you can do, which can cause issues as well, is to seek to remove him as PoA by getting your grandmother, if she is still mentally competent, to sign a written revocation of PoA and leave you as her only PoA (but this can make him go to court and then force you to be on the defensive, which can still cost you more in the long run, even if you win).