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 OP uses a lot of outdated terminology for her autistic son, to the extent of being a red flag.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.