Where does he sleep? does he have his own room? Does he have a set bedtime? Can you put a baby gate up to stop him leaving his room when he feels like it?
I'm personally against any sort of physical punishment, especially at night time when the child needs to be relaxed in order to sleep. My son (he's 3) went through a phase of sleeping poorly and bedtime was a struggle, it took around 3 nights of very little sleep, but everytime he got up out of bed i would calmly put him back into bed and tell him its bed time- made sure to say no other words to him, i had to repeat this over 50 times one night (though admittedly it was harder with my son than most other children) but i kept going and kept the consistency- i made sure to do it calmly to make sure he didnt pick up on any anxiety.
He now sleeps through the night for 11 hours every night.
I have had a routine with my son for years now, he has his dinner at a certain time followed by some play time and he is allowed to watch one cartoon, then its brush teeth/wash and bed time (if your son likes bath time then a warm bath can help relax them- i dont as my son hates bath time!) with milk.
It might be worth having a word with your mum and seeing if you can come to some arrangement over his routine, so she will keep him to it if you are not there.