It's not so much what you say as your caring about them and just being available if they need you.
My father died in 1983...and no one was there for me. I remember calling a family friend I'd known all my life and begging her for 5 minutes on the phone because I was so alone...and she said, "I'm busy. I don't have 5 minutes." This is how I learned what I had suspected; I had no one. It was tough.
Just let them know you do have time for them, and repeat it now and then, and mean it.