Babysitter's child doesn't want to stay with me?

I've been babysitting two children (4 & 7) for some time now and am supposed to put them to bed in the evening. Everything went well the first few times, but suddenly the younger one, whenever he even suspected he had to go to bed with me, started screaming and crying violently and ran out the door to his mom (she works in the same building). He told her that he didn't want me there in the evenings anymore; he'd rather she go to bed with him. He cried for over 30 minutes and could only be calmed down when we went to his mom and she told him he could play with his cell phone. But as soon as this was turned off, it started again.

I've already tried to explain to him that his mom has to work and can't come to him. I've also tried reading stories, sitting in his room with him until he falls asleep, or distracting him with a game, but as soon as he knows it's time to brush his teeth and go to bed, nothing seems to help. His mom has also told him that he doesn't have to sleep and can just wait for her in his room. But that's no use either… Do you have any tips on how I can "persuade" him to stay here and at least "wait" in his room?