Rescue plan needed!

We rescued Bobby a Cocker Spaniel, aged two years old, from a well-known rescue organisation approx 10 days ago. We were told that he had been handed over by his single owner because he was being left for 10 hours at a time and had become destructive! The only other issue we were told about was that he pulled on the lead. Our two previous rescue dogs are no longer alive, but one had had no training and the other was a severe cruelty case. Now we have a young family, Bobby seemed right for our present situation, and we believed that we would have no problems training him.
Bobby is very bright, has responded well to training and with the use of a Kong and other activities has shown no signs of destructiveness. However, whilst on the lead, Bobby becomes extremely vocal and excited when he sees another dog, which appears to be from frustration. However, I took a risk and let Bobby off the extendable to see what he would do when approaching a group of dogs off lead and unfortunately, he attacked an elderly German Shepherd. We think Bobby has been exercised on an extendable lead, and for the time being are having to use one ourselves until we are sure of his behaviour, not least his recall. (He is learning to focus on us whilst out, but this will obviously take time).
I spoke to the person who runs the local dog training class and attended for the first time tonight. Unfortunately I ended up walking out! Bobby was extremely agitated and vocal and was upsetting the other three dogs. I moved further away from them, and then out of their sight, to help Bobby calm down and focus on me, but the trainer was not happy with that. I was also not happy with the suggestion that we use a Citronella spray collar on him.
I have come home and decided to initially get some help from Dogs Today! I will also try and find a COAPE trainer because I realise that a training class is not the place to help Bobby.
We are using a Gentle Leader to help with lead control and, as I said, are using an extendable to allow Bobby some space for running.
Until we find a trainer who can help us, can anyone give us any ideas as to how to deal with his frustration/aggression on his twice daily walks?
Donna Ely, by email