Just writing thoughts out loud here. The stories do seem a bit suspicious of the mom/step dad as mom was last with him alone. Did someone mention the household was abusive? Again, not sure if an accident occurred and they panicked or if manslaughter occurred out of frustration or an incident? DId he simply bolt and get lost as some autistic children do, had he wandered before? Did he have nefarious "friend(s)" that knew where he lived & could have lured him to sneak out that night? Someone he met out while playing video games at the bowling ally?
Mom's story of that day seems like it was just a fun-filled awesome day with no incidents or challenging moments, & maybe it was, which as a mom would give small comfort knowing the last day spent with my son was a wonderful memory.
Were there witnesses who saw Sebastian out with his mom that day? Are there any receipts or documention/hard evidence to support she/he were where she said they travelled that day? Smaller towns would easily recognize their residents, conversations could have occurred with people while they were out and about.
However, I think another poster said dogs were brought in and they could not locate Sebastian's scent outside the home? That's odd. Also, it was Feb so pretty chilly in TN to leave the house with no shoes etc, which IMO would make it easier for dogs to pick up his scent? The scent should have at least went to the family car since he was in it that day. Were the clothes he wore that day in his room, suggesting he changed to night clothes, since he was told to go to bed, or are the clothes he wore that day also missing suggesting he never got ready for bed? If he did leave on his own free will or lured, he didn't take his phone etc with him, so seemed he popped out and planned to be back in fairly soon.
Hate to speculate incorrectly, as i don't live in the area or know these people personally. Seems his biological parents divorced & didn't get a long well from what I read? I'm sure police interviewed people that know them, wondering what the general temperature of the family dynamic was?