Since his grandmother requested to see him, he should go. This will be a good exercise in dealing with tacky people. Teach him how to excuse himself from a conversation that he is not comfortable with. He can say that he needs to get a drink of water or go to the bathroom, go outside for some fresh air, whatever. Do some role playing if he isn't sure how to handle it. Unfortunately, this is a skill he will need to have throughout his life. If he is asked anything point blank, give him some good answers that imply that he doesn't want to talk about anything negative about his grandma or anything about money, whatever feels natural. He will feel better knowing he has options to stay in control of his role in the situation.
Your son may even need some good questions to have ready in case he is alone with some of his relatives. He can ask things like, what did my dad like to eat when he was growing up, where do they work, have they always lived in that state, etc. Make a list of questions for him to write down. By writing them down, they will register in his memory better.
In the meantime, see if you can find pictures of your son with his grandma or your husband with his grandma so your son has something to talk about with her. Does he have any memories of her or her house, etc.? Does he remember stories of his dad growing up? That would be nice for her to hear.