3 year old misses mom at night and takes hours to put to bed

Okay so long story short, I have a 3 year old nephew (4 in February). His mom (my sister) has a lot of mental problems and is just overall a piece of shit mom. She recently moved across the US to be with a guy she met online, leaving her son. She's signed over temporary custody to me and our mom.So I was already really close with my nephew before she left, and hes always stayed with us (me and my parents) a lot. Something I've noticed, is that she left on January 1st, and about 2-3 weeks before that, he started wetting the bed. He never used to wet the bed, even when first potty training. So I think he was picking up on stress or maybe she began being more mean (yelling) at him again.Fast forward to 2 nights ago, I gave him a bath, we read books, and then we said good night and shut off the light. Usually he's really good about going to bed. He kept coming up with reasons not to go to bed (thirsty, give other aunt a hug good night, go potty, etc). Finally I think we get it all taken care of and lay down again next to him. So I'm laying there and hear him biting his toe nails (a habit he also just started around the bed wetting). After about 5 minutes of him biting, I asked what's wrong. He said he missed his mom. I think I just said I know and cuddled him. He continued biting his nails like a nervous lil wreck and would not fall asleep, it broke my heart. I think we got in bed at 9 and this went on until 11. I'm pretty sure I fell asleep before he did.So last night didn't take as long. I moved his toddler sized bed in my room. He was acting up so we skipped books and shut the light off right away. About 20 minutes after I thought he was sleeping, I hear a sniffle and then him start crying. So I got up, started rubbing his back, and he stopped crying. I asked if he wanted me to lay with him, so I laid down in his toddler bed with him. He just kept tossing and turning. Well then he says he wants water. And starting the stuff so he doesnt have to go to bed. Then he sat up and was just sitting there. I asked what he was doing and he asked me to turn the light on, and he said "to see how the room is." So like he's just trying to find ways to put off going to sleep.He ended up asking to sleep in my bed, and I told him if he's quiet and lays down he can. So he fell asleep shortly after moving to my bed. I would say I put him to bed first in his bed around 9, and he eventually went to bed at 10-10:30.So I'm just wondering how I can help him go to bed easier? In the day, it's easy to keep him distracted and busy. But nights are just really hard. Any advice is greatly appreciated, it breaks my heart seeing him like this via /r/Parenting http://bit.ly/2LUTckY