How to deal with INS's mistake

I applied my son's AP(Trave Document) last year. In Jan., we got INS's notice that asked us to show his approved I-140. He doesn't have himself I-140 because I am the I-140 owner of our family. Therefore, I attached my approved I-140 and submitted I-485 and his I-485 to INS.

Yesterday, he got a denied letter from INS in which INS stated the reason is --

"The facts presented indictae that the I-485, Application to Adjustment Status to Permanent Resident, wes APPROVED." Also, INS said this is final decision with "NO APPEAL".

This is a big mistake. We have not got any responces like Finger Print and Interview from INS since we submitted our I-485s. INS Status check function still shows that our applications will be take 900 to 999 days.

How should I deal with such a ridiculous mistake especially that INS said NO APPEAL?

Thanks.