It is known as Grandma's rules in Grandma's house. Make it clear to him that when he is with grandma he has to use the toilet. It will take a little time and much watching, but once he gets into the rytthm of it, he will understand.
he does need to learn it, though because soon, as he grows, what is sweet behaviour in a toddler becomes very problematic when done by an older child and could lead to exclusion from school. perhaps that is an approach you could use with his mother - that he is rapidly reaching an age when this kind of behaviour will no longer be acceptable and will cause socialisation problems.
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