I don't know. How much does he drink? Is is the same amount every night? Is he on an anti depressant, which his Dr may have prescribed because the Dr knew that depression would set in? Too many questions to figure this one out on a forum.
I can give you an example though of a really great neighbor I have. She is 86, and yes I give her rides when her arthritis is too severe and she doesn't feel comfortable driving. Her hubby died about 7 years ago. Every night she has her 'nightcap'. Same amount, never changes, 1 drink, that lasts her about 1 1/2 hours and she usually puts more ice in it in that 1 1/2 hour time frame. Is she drinking more then the Nat'l Council on Alcoholism suggests? Yes. Is she an alcoholic? Nope. She won't take sleeping aids and refused anti-D's from her Dr, says her 'nightcap' works just fine.
Not my place to say if your father is or isn't an alcoholic. Is he drinking more than before your mom had to go to rehab for her hip? Or are you just more aware now because without your mom there you are talking to him more often?
There is so much more to being an alcoholic than the 'amount someone drinks' or the 'frequency that one drinks.'
I am sorry I could not be of more help, but it is a question that is not easily answered.
Love and hugs,