Family Issues Support Group
Family issues is a huge range that go from minor conflict to major misbehaviour and even abuse on the part of individual members of the family occur. Whether you feel you are in a dysfunctional family or you're dealing with a very specific issue, this is the place to talk about it and find others who might be going through the same thing.
I'd email your father and air it all out. How the previous times, it was HE who cancelled. I'd also say that you will NEVER leave your son alone with him again because of his drinking.
And YES, btw, parents DO trump grandparents. If he can't understand that his paternal grandpa is leaving for FIVE years, then that's HIS issue.
Why every weekend, anyway? I've NEVER had a babysitter for my child... ever. I was a single mom for five years.
BTW- my parents are so toxic, I've had to put it in a court order that all contact with them regarding my daughter has to go through ME. (My ex once took her to see them on his time, and I almost lost it. Lied about it, too!)
This is about your dad's drinking, not about plans changing. You have to make that clear. Of course, he'll gas light and twist it to make it your issue, but that may just give you the space you need.
I think Deli is right, you should talk to your father about moving forward in the future and setting up specific dates/times for visiting and both parties need to adhere to the original plans.