I wish I had a good suggestion for you, but I don't.
My guess is there's not much you can do or say to change her pattern of favoritism. The person who might be able to help is your husband, because it's his mother who is the offender. I doubt that he could do much to change her feelings about this particular grandson, but he might be able to persuade her not to make it so obvious in front of her other grandchildren. Don't risk having your mother-in-law turn against you by complaining to her about the situation.
Other than that, I'd work on my kids. Point out to them that their grandmother loves them—always gives them birthday and Christmas presents, etc.—but that she has always had a special spot for the one who is her favorite. (Maybe he had a difficult birth, or reminds her of her own father—make up something if there isn't anything actual you can refer to.) Then I would urge them to be more aware of and thoughtful of their grandmother—write little notes to her, draw pictures for her and send them in letters, invite her to visit their school, etc. A little extra effort on their part can make a big difference in her behavior.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.