Help!  I've "adopted" an 18 month old springer spaniel - my springer, Buster, died about a month ago, the breeder phoned me to say Gerry (Buster's younger brother) needed a new home, job done!  However Gerry is the complete opposite of Buster......each time we meet another dog on the walk he lunges at them and attacks.  This is completely new to me.  He had a go at a black lab when off the lead in the park and was soon put in his place then the two of them carried on playing really well which is exactly what was needed.  However it is now making the walk a problem rather than a pleasure since he takes on anything with 4 legs barking and lungeing at them and pulling my arm off!  It appears to be worse when he is on the lead.  All his last owner told me was that he was attacked by a staffie when he was a young pup.  He may be doing it out of fear rather than domination but I don't know how to tackle it.  He hasn't had the snip! 

We are also doing our best with the gently, gently approach to getting him back downstairs, off the sofa etc.  Each time we try to remove him he wees.  It is getting better slowly but has anyone any tips on how we become pack leader instead of tiptoeing around? 

I would find a good behaviorist, He needs positive experiences NOT being put in his place, this will only reinforce the fear and make him worse.
My girl is timid and had a bad experience when younger, well a couple... and she was fear aggressive, didn't bite, just barked and lunge we use clicker training, every time she looked at a dog or was near a dog, click, reward with tasty high reward treats. Every time she barked or lunge we walk a few paces in the other direction always with a loose lead, then back towards the dog clicking and rewarding calm behaviour, timing is the essence with the clicker... :) what works for one might not work for another, it's important to find a good behaviourist, we had a vet referral  to our behaviourist :)
Also worth a vet check to see if there is anything medically wrong with your boy.

Someone more experienced will advise on other matter.

Just to wish you Good Luck :)

one of my yorkies, Dylan, was exactly like this! he's 15months old, had the snip about 5months ago, and i had to get an animal behaviourist in to help me....best thing i've ever done!

she came 3 weeks ago, and i'm starting to see a change in Dylan already, but it IS a long slow process!! i have to take him out for a walk separate from my other dogs, so i can give him 100% concentration. i've blocked off the upstairs with a stairgate, as Dylan's problem was that he thought he was top dog, even tho he's the youngest!

if you've got lots of patience and determination, you'll get there, i'm sure :-)

thank you for your advice.  I took Gerry to the vet this morning to have him checked over (made sure I waited in the car so he didn't have a go at the other dogs in the waiting room!).  I have a behaviour expert coming round in an hour........noticed this morning though that Gerry was absolutely fine running around in the park after the ball with all the dogs, even a boxer pup of 5 months and a jack russell!  It appears to be when he's on the lead.   Weed all over my bed this morning as well though when I failed to leave the door closed - he cleared the barricade I had made at the bottom of the stairs.  Don't you just love em?!