Angels are higher beings that were created at the dawn of time for the sole purpose of keeping the universe and all its inhabitants thriving. Sadly due to the free will choices of human beings its not always an easy job for our Angels. Angels also have free will choice.
There are many theories about Angels and all we can ever do is take what feels right.
It is my personal view that human souls or spirits cannot be our angels. As St. Thomas Aquinas might have posited in scholastic terms: Men and angels are not just different in degree, they are different in kind. That is, humans are not angels, angels are not humans, nor can one ever become the other. Angels are created by God in the supernatural realm (the invisible or spiritual realm), while men are created by God in the natural realm (the visible or physical realm). If a human being moves from the natural realm to the supernatural realm of heaven, he becomes a saint, not an angel.
Angels are fundamentally pure spirit beings and made up of light tones and frequency. These frequencies vibrate much higher than the human physical form, making it harm to see them. Angels can from time to time influence the behaviour's of humans to make them appear almost angel like in their conduct.
Now in an effort to reassure the human race as many people still refer to humans becoming an angel, it is very possible for a loved one who passes to come close to their loved ones and watch over them through out their lifetime. They often stay close to help them process their grief or to help them through troubled times.
They can also give messages of reassurance through the means of a medium. So to answer the question it is of my view and experience that your generous kind hearted grandfather cannot ascend to angel status. Its simply an impossibility however they can continue to do good deeds in the spirit world and by coming back to help family members in times of need.