I do believe they can go in and do motre surgery but it's no garrintee. Usually if a gelding still thinks he's a stud he has been gelded proud. When being gelded if they leave anypart of the cord this causes the gelding to still produde teststron and act like a stud.
I would definatly call a vet to see if there is anything they can do but I do believe it would be more of an exploritory surgery whit no garintee.
I would definatly remove him from the mares pasture! It is not fare to the mares to be harressed all the time and if he is study enough to mount and penitrait the mares you are risking them mares getting bladder and uterine infections.
Cathy C