Uhoh... I mean his eye looks okay. But the white gunk could be a sign of something respiratory that comes and goes with stress and make birds lifelong carriers.. and then added to a previously runny nose. That right there, would be quarantine fail for me.
Sorry, I'm a little obsessive/compulsive with an extreme attention to detail and a WAY over thinking brain that spins 1000 times per minute.
I don't mean to cause alarm. 
Your goals may be different than mine and since I breed my birds, even as a hobby there are risks I just cannot take. But you probably saved his life, please use your own judgement! 
If he had food it does sound like he was dumped.
I wish people would educate themselves and look for a better alternative. Those facing job loss or foreclosure or maybe worse that I don't know about... Maybe that was their attempt at a best. I hope they get help, because they obviously have issues! 
Since you seem interested in learning and I am too.. His legs (shanks) might be considered willow color?? I am still learning to identify things I haven't already bred for.. and I know someone will quickly call me out on it if I'm incorrect!
His skin might be yellow, the underside of feet is a good place to tell. How long have you had him now? Sounds like he got lucky to find you!
