It's tricky to try to diagnose anything online--so much depends on how he's reacting, when he's reacting, his body language--no one can see any of that when they're not there with their eyeballs staring at the dog and his behavior, seeing how you folks handle him when he reacts and watching how he responds.
With a really reactive dog, one place to start is to keep him far enough away (from another dog, if that is the trigger) that he never gets aroused, and then to work on attention exercises, until you can slowly get closer and still keep your dog focused on you. His reaction may be situational...maybe a class would help. But it would need a great deal of care, advanced warning to the class instructor as well as a lot of knowledge from that instructor, to make sure that the reactive dog isn't actually getting reinforced in his behavior. And of course, you have to think of the other students and whether you are interfering with THEIR learning experience.
So much just depends on the severity of the dog's reaction and how "set in stone" it is in terms of knowing what direction to take to work with the dog.
His age does mean that he may be still in that possible teenage fear stage, which I'm sure doesn't help in determining how he may develop from here. A tricky situation all around, which, like you said, ECIN, will in all likelihood take a lot of work, long term, and maybe always a little extra vigilance to deal with.
I don't think unfixable though, given that his behavior in other venues appears to be exemplary