The 23-year-old was injured after coming off a quad bike at Carroway Head Hill, near Sutton Coldfield, shortly before 9pm on Tuesday, June 15.
He remained in hospital this morning.
A spokesman for Staffordshire Police said: "Officers investigating a road traffic collision near Sutton Coldfield are appealing for witnesses.
"A 23-year-old man was left with potentially life threatening injuries after coming off a quad bike shortly before 9pm on Tuesday, June 15, at Carroway Head Hill.
"He remains in hospital at this time and his family are being supported by specially trained officers.
"Officers from the Staffordshire and West Midlands Serious Collision Investigation Unit are now urging anyone with information or dash-cam footage to come forward."
Anyone with information should call Staffordshire Police on 101, quoting incident number 741 of June 15. Alternatively, witnesses can call Crimestoppers anonymously on 0800 555111.