Give dad a hug and tell him that you're glad that he's still here. I lost my dad in 06, my mom last June, and my grandma last month. It's too late to tell them when they are gone...
It's a bad feeling when they are gone and you realize that you are now alone. You can't pick up the phone and talk to them any more.
Be there for your dad, and help him get used to being alone. I saw my neighbor across the street go through this when his wife died a few years back. Then to top it off, he lost his dog a few months ago. He's all alone in his house now. I go over and talk with him for 15 - 20 minutes when I see him out and I have the time. He thanks me for coming over and appreciates even a few minutes of chat with someone. Dealing with the lonliness is very tough, especially when they were together for so long...