It sounds like the siblings aren't in the same house? I would treat it just like you would if it was cousins that he was treating differently. And unfortunately, there isn't much you can do about that. I remember Christmas coming around and I got a $5 hair dryer, and my cousin got a beautiful English saddle, and I was the one that rode. Later, my grandmother saw an add for the hair dryer ($3.50), and was upset that she didn't see that deal before!
You have told him how it makes your son feel. Is your son old enough to tell him himself? Either way, if the grandfather only sees him when your husband makes the effort, stop making the effort, the grandfather has made his decision. Just don't let your son know how wrong that is, and let his life be full without him.
UPDATE: I hope that my answer wasn't referred to by others as "cutting him out of your life". My point is that it is up to him to make the effort, not you. While I had the grandparents that I mentioned above, I also had grandparents that seemed to be constantly fighting with my parents, but they would still call, ask to see us, talk to us on the phone, etc. They were able to maintain good relations with us even when they weren't talking to my parents. Unfortunately, I have lost both of them, but loved them dearly. My father sees a lot of my niece, but has seen my son twice, partly distance, partly he is not talking to me (long, stupid story). I still share pictures with him, and let him know that he is welcome to see his grandson whenever he wants, but he is going to have to put in the effort, as he isn't satisfied with us seeing anyone else if we travel all the way to California. My son's other grandparents live in the same town, and make no effort to see our son. My husband was making a huge effort, but we finally decided that they know where we live, they have our phone number, and have been given an open invitation. The ball is in their court. My son's life is full as it is, but I would love for there to be more family around.