Ive seen a few geldings actually "do the deed" before. If he's not a rig, than I think you are right - most likely gelded late. I see this a lot with recehorses who were not gelded til the three of four year old year. Sadly it is often such an ingrained behaviour that they never get over it. We have a gelding at the farm who can NOT be turned out with others for this reason. If you can muster the funds, get the blood done just to put your mind at ease.
All I can suggest is to do what you did before and hope for the best.
Sorry for you being in this sticky situation - it sucks!