Maybe give him a wash out. Wear rubber gloves, use warm water, and either a very mild soap or a lubricating gel. The skin folds can get full of muck and start an irritation, which brings on swelling. The joys of owning a gelding.....
has it been extra hot? has he been moving less since moving on the fresh grass? you might find it goes down with movement.
my gelding did get stung on his once too, so some antihistamines were needed