Several questions for thought;
How long between the argument and his leaving the house at 11:15, if he contacted someone that he was coming over (school friend etc.) how did he do it if parents had taken his phone?
If the argument was earlier in the evening did he make arrangements with someone to pick him up out at the end of his road? If so, who might do that?
He has lived in Iowa and knows the weather and he is a smart kid, it doesn't make sense that he would dress as lightly as he did and head out into the night with no plans on where he was heading.
If, in fact he did contact someone by phone, wouldn't there be a record of it for LE?
Are there other extended family in the area,(grandparents, aunts and uncles) that he knew would welcome him and he set off in the direction of their home.
Was he active in his church and knew how to get in there for the night?
It looks like he lived in a developing neighborhood, are any of the homes that are not owned yet, heated so he could be there.