Grandpa:   Mom's the one who needs to address the behavior.  You can't fix him for her.

Your grandson is not used to being with lots of family.  At home, it's just him and his mom.  When they are with you, she's inter-acting with other family members, and he sees a new side of her.  He's not comfortable with this, and he's challenging her.  Respond with love and reassurance, not confrontation. 

Gather some quiet things in a small tote (crayons, paper, small toy car, puzzle book) that only come out when he needs help behaving well in the restaurant.  Invite him to come sit by you and give his mom a break so she can enjoy a few moments conversing with other adults.