I think that far from being supportive, my attendance would be a distraction. I haven't been inside a kingdom hall for 12 years, and I fear that it would divert attention away from what should be my dad's day. He's a big boy now, I'm sure he can get through a public talk without my support.
I do wish him well, but the thought of being in a kingdom hall and listening to (even a mere 30 minutes of) the watchtower's propaganda makes me feel quite unwell, and it would probably make me feel even worse to hear it coming from my own dad.
More importantly, my being there would give my family false hope that I may come back to the religion, and I don't want to do that to them; I don't want to toy with their emotions. It would benefit neither myself nor my family for me to go, it would benefit no one.