My son said my mom hurt him. ‼️TW sexual assault ‼️

My son is 2. He says all kinds of things. Sometimes I’m assuming things he dreams about. But the other day he said grandma hurt me and grabbed his penis. He had just got out of the bath and always try’s to grab so I don’t know if he meant down there. My mom was raped and abused at a very very early age. I could never see my mom hurting him. I asked her about it and she said the only thing she could think of was he had a fuzz stuck to him while changing him and when she got it off he said owe. Idk what to do. I will never not believe my child, but I also don’t think my mom could ever hurt him like that. She doesn’t even spank him because I don’t spank or use that kind of punishment. She watches him while I’m at work. But my sister is also in the house with her so they aren’t alone. He has never said anything like this before. Any advice??