Hi,
My 3 year old daughter went through this phase for a month, so I can understand how difficult it is to try to instill some structure (important!) and routine (important!) in a child's life when the other home is very permissive.
I don't necessarily agree that your house is necessarily the un-fun home if it is the structured one. My daughter's mom reports that daughter hits her, has tantrums, fusses at bedtime, complains about eating, etc.
We don't have any of these issues because we've instilled some expectations mixed with making even routines fun.
If you haven't start building some nurturing routines prior to bedtime. The bath, then snuggling together on his bed to read some stories, then tucking him in and doing a special final thing.
If he needs a nightlight, or any other reasonable sleeping aid, of course it doesn't hurt.
Here was the coup d'etat for helping my daughter through the difficult bedtimes she had for a few weeks. I took her markers, drew a picture of her smiling in her bed (I'm not an artist, so it was just a smiley face with her color hair in a bed that had a blanket like hers). Beneath the drawing, I drew three empty boxes. I told her that every time we had a happy bedtime, she'd get a star in one of the boxes, and when the boxes were full, we'd go to Toys R Us to get a little toy that she earned. I taped that sheet to her closet door, so she could see it.
The first night, there wasn't any change. In the morning, she wanted a star. Nope, sorry. That night, we had a happy bedtime (i.e., no complaints, and when I checked on her 10 minutes later, she was asleep).
After quickly getting her first toy, she progressed to a six-box chart (i.e., make it harder to earn). After the second time we got her toy, she started losing interest in the star chart, but the new peaceful routine had stuck.
Of course with each toy, we made a big deal out of it... it wasn't just some gift... it was something she should be really proud of because she earned it.
We didn't want to taint her choice in toys by putting a price limit on it (which she doesn't really understand), so on the first one, I took a little checkbox and said she could pick whatever fit inside of it (i.e., figuring it wouldn't be too expensive). The next time, we took a slightly bigger box, because she had more stars.
You gotta find what motivates him to help him convince himself that he really doesn't need mommy.
Best,
DD