I'm a mother-in-law and I understand his staying out of the picture in regards to his son's and your separation. He is probably limited in his ability to express compassion, especially in a situation that may seem to him to parallel his own past experience. He may feel compassion for you but be unable to express it. He may think that a card is a way of acknowledging 27 years of marriage and is showing you support. You have stuck with his son for 27 years! That is a long time to stay married! That may feel to him like quite an achievement, especially in light of your current situation, and that you deserve to be congratulated.
Remember that most men in that generation were taught to not only not show feelings but to also not express them if they did become aware. I go back to the idea that your situation may be too similar to his own past situation for him to deal with it.
There is no comparison between his reaction to your situation and yours to his 20+ years ago. On the surface they appear similar but because of time alone, his and your and your husband's reactions would be vastly different. You were young then. He is old. He was starting a new relationship and you had years ahead of you to influence how you reacted at that time. Even as adults, we tend to want our parents approval. You had a young family, most likely, and wanted him to continue being a part of your lives. As an older parent, he may have already stopped building a future and if he's focused at all he may be focused on letting go of this world. I'm starting that process at 67 even tho I'm also focused on being happy for the next 25 years. Because you were young, you also had energy and the support of your husband, his son. You didn't mention his wife. Does he have support from a woman to help him understand how you feel?
I'm also guessing that, since you've been married for 27 years, he has let go of needing your or his son's approval in the way that may have been present 20+ years ago. For you, a separation from your husband, after 27 years of marriage, does put you back into feeling a bit like a child again because you are feeling vulnerable. You are wanting approval and connection with parents. I hope that you are getting that from your parents. If they've passed on, I know that this situation may be even more difficult for you and might cause you to want support from your fil.
You talked with your fil when you separated. Does that mean that the two of you talk regularly, even tho possibly not often, and have a relatively close relationship? If so, I'd visit with him and ask him his intentions. If you've been able to share your feelings with him over the past several years and felt that he accepted them or even understood you, then I'd tell him how you feel while giving him the opportunity to explain himself and reassure you that he didn't intend to be mean.
Or, if you see him from time to time, arrange to spend some time with him so that you can sense first hand how he is probably feeling but is unable to express those feelings to you.
But if you don't have an ongoing, sharing communication type of relationship with him, I'd ignore the card it's self. I would find a way to deal with your feelings about it without involving him. Perhaps talk with a friend about it or journal, writing down how you feel and process those feelings until you're comfortable with the idea that whatever his reasons you can still love him.
It's important for you to find a way to resolve those negative feelings so that they don't continue to color your relationship within the family. If you feel hurt and angry towards your fil those feelings will spill over into your other relationships such as those with your children. Especially with your children. Children are usually much more sensitive than most adults and even tho they may not know how you feel they will sense that you no longer like grandpa.
Ignoring how we feel does not work. Ignoring does not make the feelings go away. They just go underground. If, from past experience, talking with him has a good potential of clearing the air and helping the two of you to continue with a good relationship, then talk with him. If you're feeling hurt based on past experiences with him hurting you then definitely do not talk with him but find a way for you to process and let go of the hurt feelings.