Okay...guys are different from girls, and what might be concerning for a girl maybe not so much for a guy.
Has he been mating a lot recently? Or suddenly stopped mating (as in put in isolation after mating a lot?)
His swollen vent may be a natural reaction to, well, uhem, masculine swelling. That might be causing some clogging in the vent area.
If you are certain it is not a reaction to mating a lot (which can increase the organ size), or not mating after mating a lot (which could keep the size inflated), then it could be his diet causing constipation.
The other thought is check your hens...do any of them have especially dirty bottoms? If some of your hens have vent gleet (an overgrowth of yeast), that infection could be passed along to your fella through mating.
His vent would likely look red and may even have some cheesy appearing discharge. Your hens will look a mess with very drippy, messy bottoms that often smell bad, and red swollen vents if the infection has gain hold.
You can google vent gleet here on BYC for measures...often cleaning and drying the vent, applying tinactin spray, and supplying probiotics or yogurt in the diet with apple cider vinegar in the water can go a long way to help put the flora into balance for mild to moderate cases. More severe cases require anti-fungals such as nystatin which a vet has to prescribe.
My thoughts,
LofMc