Did you post a found add in the newspaper? It is possible that someone lost him and he wasn't dumped. Think how you would feel if your snake got loose and you never found it. Did you take into the vet or the pound to see if it is micro chipped? I would wait at least 2 weeks before getting attached to it. I would also call around to pet stores and see if there are any herp shows around and contact them. Sorry for sounding harsh but you assumed he/she was dumped, but I have had enough snakes escape (granted never anything that big). Shoot at least give it a week and post some flyers at the local pet stores.
Thank you for taking it in and caring for such a beautiful snake.