As Doganjo says then. Once he is 'excellent' at his lessons when on his own with just you, then add a distraction. To start with it could be as simple as your oh holding your old spaniel on lead in the same field as you train McCoy.
If the spaniel is obedient then go on to use him in the lesson. He heels, sits stays while young dog has to just sit and wait. Sit them side by side and give stay command. Recall them to you one at a time.Other must not move. Heel both off lead. Give sit command. Send individually to play using whatever is your command for that.
You could then ask a neighbour to do the same with their dog.
It's all about adding those distractions and expecting the same level of behaviour from your dog despite what is happening around him. Built up gradually and positively of course. He will learn to ignore the distraction and focus on you.
All helps with the control you need when you take him out into the bigger world. Use what's around you. If he is excellent on his own it's probably time to push him to the next level, with the distractions.Youll push his obedience to the next level and this will help with the other problems, whatever there cause is and make him into a dog that you can take anywhere.
Good luck.