If it only started 10 days ago you may be able to put it right.
Do you know anybody with a quiet laid back dog that you could walk with.
If he hadn't shown any aggression before, it is probably fear based, so getting his confidence back is important.
You don't want him doing the same to other dogs
Is he neutered, as this may help because he is at an age when he is maturing.
I had something very similar happen with Alf, and although he isn't aggressive, I can't trust him around black labs
The trainer is obviously very important