I agree that horses are weird. My gelding became rather obsessed with an OTTB mare a few years ago when she came to the barn where I was boarding. Part of his problem was he was also dealing with anxiety and (found out much later) ulcers. But it was odd that he really focused on HER in particular. They were never out together, but sometimes in adjacent fields and at times he’d run the fence line (ignoring his pasture mates) if she wasn’t close enough for his liking.
Moved to a different barn ON THE SAME LARGE PROPERTY and she moved to the other side of the state. He got along with other horses at the new barn and was fine being alone. His OTTB girlfriend came back to the second barn where my gelding lived and he ignored her completely.
Now they live together in a 1+ acre pasture and 2-stall barn and are pretty much joined at the hip. He’s fine for being taken away from her for riding, etc. but now SHE cries for HIM whenever he’s taken out. Her owner says she’s never gotten attached to another horse like that. In their little set-up, if I close him in a stall and take her to the next stall (they can see each other…just a half-wall separating them), for the few seconds my gelding can’t see her as I walk her to the door of her stall, he’s nickering and fretting for her…fine once she’s in the stall. She, on the other hand, has no issue being closed up in her stall and not being able to see him. Go figure! They adore each other, except when he’s bossing her around and she’s making ugly faces at him behind his back, LOL. I truly think they recognized compatible souls in each other from the first moment they “met” back at the first barn.
Hopefully your fella will get over it. Some horses just get worked up by newbies. I’d let him be as obsessed as he wants on his own time, but I’d be pretty clear that the shenanigans are not appreciated when he’s in your company. He’ll probably get over it soon.