If it is just his sheath and another ventral-midline spot, NOT any filling in his legs, came on suddenly yet he is acting normal (not droopy or having trouble peeing), and assuming that his heart/liver/kidney function are normal.... then I would tend to suspect either a bug bite/sting or a kick. Things like this on the underside of the belly can sometimes swell up a *lot* and stay that way for longer than you would expect, just cuz the fluid has nowhere to go just from gravity so must wait til the body can slowly resorb it.
The other possibility that comes to mind would be an infection in the sheath (you can't always SEE a focus of infection). Bloodwork should tell you about that, too (I'm sure your vet is having a basic panel done that includes a WBC count)
If it were me I would NOT try to wrap it unless specifically instructed by the vet, would continue to give him as much turnout as possible, and just keep an eye on him. If it is not something real serious (the things your vet is checking for), and it is probably not, then it will very likely resolve on its own.
Good luck, have fun, hope he's feeling better soon,
Pat