It's possible that people are overly concerned with "normal" as opposed to what works. One area I've seen this in first hand is sleeping arrangements.
...how do we stop his closet-sleeping?
Offering a more favorable option is the best way. That respects the child involved instead of striving for a more "normal" arrangement. For reasons your child (who is wonderfully unique) cannot explain, he feels better in the closet. It may be that he feels safer; it may be that it's darker; it may be that it's cocoon-like; it may be that it's quieter. Who knows? You and his mother are the best people to guess, experiment, and find a mutually agreeable option.
If it's noise, maybe a white noise machine would help; if you already use one, maybe it's too noisy. If he's uncomfortable in open spaces, maybe you can make up a small tent over his bed and see if he likes that better. If he's fallen out of bed, maybe try with the mattress on the floor. If he's afraid, you can try putting him to bed on the floor in a corner of your room. Whatever works for you parents and the child is good. But the child needs to feel satisfied with the arrangement as well as you do. Make sure the child has a comfort object or two as well (e.g. a plushy and a favorite blanket).
When my kids were little, they each went through a phase when they would awaken almost every night and come to our room. Their father would insist on them sleeping in their own beds, which led to middle of the night power struggles usually involving strong language and lots of tears, which I was not OK with. Fed up, I bought thick foam pads, covered them with a sheet, pillow, and a blanket or two, and put them on the floor on the periphery of our bedroom. The deal was, if they woke up and wanted to sleep in our room, they could just help themselves to a bed on the floor, and just like that, the problem solved itself, no more power struggles, no more crying, unhappy children, no more (sometimes) hours of lost sleep. It worked beautifully, and each child outgrew the bed-on-the-floor-in-our-room phase. Even their father thought it worked very well.
To me, the most important thing was that my kids felt heard/respected, and felt loved. They all seem to me to be normal adults in spite of their sleeping arrangements. So just keep experimenting until your child contentedly and safely stays put in one place all night without disrupting your sleep. That's when you've found the perfect sleeping arrangement.