He is required to have an interview.
His aunt can accompany him to the interview.
His father needs to sign a notarized consent letter allowing his child to immigrate. The US Embassy will not grant a visa without this letter.
A K-2 can only be issued within one year after a K-1 is issued; not when the K-1 arrives in the US.
It would be faster for the USC stepparent to file because this would be treated as an Immediate Relative case and take about 6-12 months. You should file the I-130 as the USC stepparent.
An LPR parent would petition the child in the F2a category where it currently takes about 18 months for a visa. It could be longer.
