I'm not entirely convinced that's pigeon fever.. as it isn't affecting both pectorals. That doesn't mean it couldn't eventually. Hopefully somebody else has seen examples of pigeon fever and can say yes or no.
Pigeon Fever doesnít necessarily have to affect both pectorals. Had a possible case over the summer that only affected one side of the chest (I say possible because the abscess drained before my vet could collect anything for testing so we were still up in the air about whether it was Dryland Distemper or just a regular abscess.
OP, how long ago did you notice this? My gelding started showing a lump in mid-July. I thought maybe it was a fly bite at first since it was pretty small, but as I do self-care, Iím around him twice daily and really noticed it getting bigger on the left side of his chest the second week or so of July. The my vet just happened to be coming out for another horse so I had her look at it. She thought maybe it might have been scar tissue or something, but told me to measure it and keep an eye on it since it didnít seem to bother my gelding. His vitals were all good. Appetite fine, no abnormal temperature, went out and had fun riding. He was just his same old self.
About mid-August, I noticed his lump seemed bigger and a little ah Ö squishier. My vet came back out for fall shots and took another look. Started throwing out possibilities of cancer (not reassuring) if it got bigger, but she wasnít too horribly concerned. It started looking really funky around the first week of September, when the skin turned a sort of funny color, and he lost hair around the lump. And then I noticed two little scabby bits that were where the abscess eventually broke and drained. Had my vet out right after the abscess drained, but we couldnít get the pus or anything if there was some to test for possible dryland distemper. She tried flushing it, but it was pretty clean at that point. If it was pigeon fever, he recovered well, and was back to normal relatively quickly.
Itís hard to say just by looking at that lump on your gelding if it is pigeon fever, but I see youíre in California like I am and the bacteriaís in the soil here, plus flies can pass it along, and we never really have a no-fly season anymore, especially when itís hot in the fall. If heís not running a temperature, your vet probably wonít do antibiotics (mine didnít) because usually antibiotics make it harder for the bacteria to go away. Your vet might lance it, but sometimes that does more harm than good because of the possibility of infection. Itís really up to the vet what they choose to do.