Sounds like nervous, fear aggression, probably stemming from his encounter with the two Jack Russells.....what is it with Jack Russells in pairs? There's two get walked together in our village & they're a nightmare, I dread meeting them if I'm out with any of mine as the pair will lunge at anything on four legs.
Should think he's come to the conclusion that he's going to shout at other dogs first to stop them having a go at him.
If you can get him to any kind of class where he can socialise with other dogs in a controlled environment you will find it a big help. Obedience classes, ringcraft classes, agility, anything will do where he can meet other dogs.
Meanwhile the best thing to do when you're out walking with him is to anticipate other dogs. keep your eyes open for dogs coming into his proximity & distract him before he becomes aware of them. Take something with you he likes, a squeaky toy, special titbits, anything he really likes. When the strange dog is getting near, but not too near, turn him to one side, get him to sit & focus his attention on you. Talk to him, pat him, give him treats or squeak his toy, just keep him focused on your face while the dog goes past. If he kicks off, just say NO very firmly & try to get his attention on you again. Every time he ignores a dog going past make a huge fuss of him before you continue on your way.
It makes a walk last at least twice as long, it takes patience before he ignores every dog, & might not always ignore every dog, but it will improve his behaviour eventually.
He can't be worse than one of my Skyes was, she was dreadfully nervous as a puppy & was a nightmare to take on walks, but with this method & fortnightly ringcraft classes she has become almost normal on walks.
This is a dog who loves to bits all the other dogs I have, sleeps curled up with one of the Yorkies & thinks the cat is her mother, but hated with a vengeance any strange dog she met on walks.
xx
Meggie