Can POA be overridden?

Alva is correct. Blood thinners can cause other problems. Alva can contradict me, but blood thinners prevent clots, not sure if they dissolve them. From what I quickly read, clots are complicated.

I would think your Uncle talked to the doctor and weighted the pros and cons. Your grandma is 92 with Dementia and lung problems.

Is he requesting palliative care and hospice? He may be fulfilling the wishes of your grandmother. What ARE her wishes at this time?
If you wish to make other decisions, as caregiver you may wish to apply for guardianship. Understand this can be an expensive process, and you well may lose as your Grandmother assigned this to her son.
As an RN the most and the worst complications I saw were do to blood thinners whether given in facility or in the home. They cause bleeds which can occur anywhere from brain to toe. Your grandmother has the right to not have a lot of heroic care and intervention if it has been her lifelong wish not to have same. Her POA may be acting in her best interests and according to her wishes.

Is your uncle also her medical POA? Whoever has that will have the say as to what kind of medical treatment is allowed and what is not. And no the POA cannot be overridden unless you go to court to apply for guardianship over your grandmother, proving that your uncle is not keeping your grandmothers best interests in mind. Good luck.