Changelings usually present the same as autistic folks do, some folks even theorize that we are one and the same. It'd be pretty weird for a changeling to appear neurotypical, tbh. OP uses a lot of outdated terminology for her autistic son, to the extent of being a red flag.

My guess, as an autistic person myself, is that the fae saw that he wasn't being cared for as a neurotypical child would be, and they saw the opportunity to take him back and did. I think they released the human child, but another entity saw a chance to get close to OP and did. So now OP has this entity in the house, and the human son was found a bit farther away.

Pity, cause changeling Parker sounded like a fun kid, probably would've went on to discover a new species of bird or something. But better to be with those who accept him.

Silver lining: Your dad was pretty awful, so post-mortem his soul probably went to hell. And you just so happen to have a boyfriend with an employee pass. As a sidenote: Where has your family's Domovoi gone, that your family isn't protected in your own house? Your family seems to take great care in preserving the traditions and protecting the spirits, and it doesn't sound like your mom would've let your dad banish Dedushko. I wonder if the red thing is suppressing him somehow?? You may want to leave out some cream and apples and honey crepes for the Domovoi

Dr Thompson seems sketch, you should try seeing someone else