I had a grandfather that told me that he planned to leave his legacy to his children, and by extension his grandchildren. I didn't have the heart to tell him that my dad and his wife would never consider passing it to us, but rather to charity when they die. He passed, and left us grandkids some money, but I think the bulk of it is going to my dad and his sister. Too bad. I just didn't think it was my place, but I was not his child.
How old is your dad? If he is in his 80's or 90's I think I wouldn't say anything, you will probably be getting it sooner than you want. If he is in his 70's I would broach it as "You might want this money for a rainy day, so why not grow it in case you live longer than you think and need to live on more of it".
My aunt lived to be 91 and when she was 88 or so, she told me finances were tight and said "When we saved for this time, I just didn't think I would live this long" She lost her own mom young and her sister in a car accident, so she didn't know how the genes would work!