I just had that problem...but not with the gelding, with the MARE! She's like your guy, normally sleepy, docile, and quite. Day before last she was dancing around, nearly ran me over and acted like a total b*tch. Called to 'her boys' and was generally idiotic.
I had the rider get off (I was walking her to another pasture to get away from the bugs). Whacked her across the muzzle to keep her from running me over, then made her feet move. I lunged her maybe 10 times in each direction at a trot and easy lope. It really helped focus her mind and get paying attention to me. She then rode out very quietly, walked very sweetly, and was her normal self again.
I would go out and aggressively make him lunge. Make him uncomfortable in his own area, in sight of the mare, make him work and huff and sweat. Then lead him away as you normally would. The second he starts up again, make those feet move!
As for catching, once he's respectful of you and your space and doesn't have his head in the clouds, just bring him a cookie out and remind him who really loves him ;)
Wait! I'll fix it....