I had a gelding who was a sweetheart in every way, and got along well with other geldings alone etc. Naturally as a mare owner, he was paddocked with my mares, and before I got him he was the only gelding with a herd of mares who put him in his place. Apart from him giving the odd attempt at my alpha mare who settled him back down, he was fine.
However, once I sold her and had just my other mare and him, he was made the leader as Honey doesn't assert herself in the slightest in herd dynamics. The place I board at brought in a gelding and put him in the free land next to mine and that was it - he was completely different. He made sure the other gelding (in his own paddock, own fencing with about a metre gap between our paddocks) knew that Honey was his. He would snake his neck, bare his teeth, run up and down the fence as a barrier between them, make awful sounds, kick his legs and front foot defiantly if he deemed the other gelding as "too close". He turned into a nasty monster over my mare, who frankly, didn't care. However, he also laid into Honey, and his assertive behaviour became micro-managing. If I separated him from her he would challenge fences he was normally frightened of, break her out into longer grass - the works. Then he started ripping the rugs off her back - I knew he needed to be taken away from the gender differences, be either alone to get his head back on straight, or away from mares at least. I ended up having to sell him and he's now with a gelding and back to being a complete lamb. He was lovely to handle in every way, just in the paddock with Honey was he terrible with his reactions to the gelding and Honey.
Since this episode, I've sworn that I won't ever mix genders again. It's just not worth it. There's a gelding who roams the property (his owners don't put him back in his fencing which he constantly breaks out of) and he winds the mares up enough that I have to fence them back. That is enough to renew my opinion that mares and geldings get too attached in different ways and in some instances it gets ridiculous and dangerous. I would never personally own another gelding whilst I have Honey (and our other mare Seoul), unless I could guarantee they would be separated at all times. The stress is just not worth it.
Don't wait and do something about this now. My first attempt would be like the above, remove him for some "time out". He probably will fret and you will also fret about what his reaction might be! If he settles, good. If he doesn't, then you're going to have to think of a solution, or sadly move out the mare and foal. Some geldings are proudly cut, or just have stallion tendencies, like my gelding above.
♥ Seoul Searchin' for the Lovebug ♥