At 6 year old he may not like being wiped he may want to be in control and left alone in the bathroom he is acting out for a reason .
I think that is a really good point...
It may be a good idea to laoow him at 6 year old to wipe himself and use the bathroom alone ..its possible some anxiety has been made and he is avoiding the bathroom .Does his mom take him to the bathroom aswell ?
From what I understand, boys do very strange things during this stage - which also freaks parents out (I know how you feel!). I had some interesting conversations with his pediatrician and a therapist, did some reading (which is not always helpful) and learned that boys (idk about girls) are learning about their bodies, exploring, and learning. He might only be doing it at his mother's house because it's a "safe place" for him, where he feels the most comfortable. Or maybe he's acting out. He might not be afraid when with you simply because you are there. Maybe he's conscious of the inappropriateness of what he's doing and knows that it's not socially acceptable, so he doesn't do it at his grandparents'. There could also be something wrong. Or, maybe he's afraid of the toilet. Any changes in his life or routines recently? Lots of possibilities, you should talk to your pediatrician and maybe think about talking to someone especially knowledgable in the field of early childhood development. Good luck! Whatever it is, I'm sure he'll be fine.
Has there been any plumbing problems recently? That may be a reason, also my son use to flush things like his toys down the toilet and caused it to overflow on numerous occasions.