Stallion behavior in a gelding

It's been 10 years since I got Mag. I've never met a horse like him. He's Jeckyll and Hyde. 

More submissive behavior than I've witnessed; he "gulps" vocally if I move too quickly by him. 

More dominant behavior than a gelding should have - sheath dropped, neck arched, nickering to humans. 

I have only seen him flirt with Bellis once when she was in heat (and I have a huge YouTube following in South America from what should have been an innocent video). 

We have no baby mules here. He cannot be proud cut.

My trainer wrote me a report of her time with him this week, how he continues to display stud behavior after the lesson is over. He's fine at the start and during the lesson, but on releasing him into the pasture, he starts posturing and nickering and stomping. 

What is up with my strange horse? Here's a video of some of their liberty work together: Liberty Work