This was addressed by the BIA in 2011. K2's are eligible for adjustment of status as long as they entered the US with their K2 visa before they became 21 years old, and their K1 parent is eligible for adjustment of status based on the K1 (they married within 90 days).
http://blog.mikebakerlaw.com/2011/07/bia-addresses-k-2-age-out-eligibility.html
The OP can adjust status but ONLY based on the K2. He cannot adjust status based on any new petition filed by his step-father because his mother married his step-father after he was 18. He cannot adjust status based on any new petition filed by his mother because she's not a US citizen. His mother must adjust status based on the K1 in order to secure his eligibility to adjust as a derivative.
12344 reacted to JimVaPhuong in K2 out of status HELP help help