he is aware that he needs to go to the toilet - in fact he wakes up, but then chooses not to go to the toilet.
If this is definitely the case, that he does not want to get up at night, then you can probably teach him that it is not acceptable in *your* house. I'm sure he has different rules at mum's house to dad's house anyway. Keep going with the alarm at your house if you're happy to, he will learn that he has to get up at your house.

Otherwise there may be some psychological reason why he's not getting up. Is he frightened of the dark or something like that? Did something happen at mum's house at night which means he doesn't like getting up in the night?