The dad still said happy birthday and I love you only to get that text back. Older folks are exposed to a lot of propaganda and many are brainwashed (the hint in this case is the "God bless you"). Without more context I'm hesitant to call him an asshole.
Based on this small exchange I'd be more inclined to say they are both assholes.
It's just sad that they can't look past this and love each other regardless.
How much exposure do you have to these bible people though? I mean the dad was taught that even being gay sends you to hell.
I'm from a small conservative town and it's hard to fault the brainwashed people either IMO. A lot of them are good people who have been severely manipulated.