Mike Miller gave you the straight scoop about installing the rivet prior to sewing, but lately I have been using a little bit of 5 minute JB Weld inside and outside the hole on the clip instead of the rivet. Nothing shows either inside or outside the sheath and it will stay 'til hell freezes over.
From what I see in your photo your option now is to install the rivet and then either fully line the sheath or put a thin leather patch over the rivet on the inside of the sheath using really good contact cement, or the obvious.....start over. Good Luck!
Paul