Vet's thought was that we are just at the beginning stages. Most likely Strangles, perhaps a version of Pigeon Fever. I've isolated him and grabbed a quick vaccine for my other two, although it may be too late if they've been exposed.
As for now, he's on a bit of Banamine and only time will tell!
Gotta love a good, nasty case of Strangles.... NOT! Although Strangles isn't fun, it's a whole lot better than something that's life-threatening!
Thanks for your input and sluething skills!