4 month old kitten poops bed

Hi everyone, I really need advice as I'm at the end of my knowledge. I adopted this adorable, sweet runt of the litter tomcat. He's incredibly small still, around the size of a 10 week old kitten, despite already being 4 months old. I'm at the vet regularly because he had fvr. Aside from his size and the fvr he's healthy, happy and content and gets along incredibly well with my older cat. But here's my problem: He won't stop pooping the bed at night. No matter what I do. I now have 4 litter boxes, they have access to toys, water and dry food at all times. And I play with him a lot. I've tried deterrents, I tried feeding him on my bed, I tried putting him in the litter box before bed and so on. I deep cleaned my bed multiple times now. The vet says he's fine, nothing's really wrong, so it's not that he's missing something. What do I do, except closing my bedroom door at night? I want my cats to have access to me at all times and I hate closing my door at night. Please help me I don't know what to do anymore :( Thank you so much!