Firstly Hi everyone newbie here , well where do i start i have had a impeccably behaved dog for the last 3 and half years we have the odd mishap but on the whole he has been fantastically behaved , he has lived with a number of dogs as i have fostered for rescues and each dog that he has been with he has been wonderful a pleasure to have and own we have always walked with friends and gone to places where he has been socialised very well . Well a few months ago i had a big walked planned for charity and i had to find somewhere for my boy to go why i was doing this long walk (62miles over 30hrs ) well we found somewhere and everything was fine the lady having him i knew well and he got on with her boy great best buddies :) , she then told me a few days before the walk she was also having someone else's dog that we knew and i said to her it maybe a bit much as it would be quite a few dogs in a new place and she didn't have that many dogs at her house in one time , well to cut along story short why he was in her care he got in to a scrap with the other dog that she had in her care well since this we have not been able to let our boy off the lead if you do he gets real hyper his whole body gets ridged and he starts looking around for other dogs , he also was like this on the lead to start off after the incident but with lots of corrections he is no longer taking any notice what so ever on the lead , question is how do i stop this he is not that trustworthy dog anymore , well at the moment atleast . Your advice will be greatly recived . 

You need to get a trainer in for some 1-2-1 help and tofind some answers for you. I would have a chat with your vet for someone local to you. 

Did the person looking after him say what had happened or what the build up to them fighting was.  He might be frightened of being attacked with his behaviour or like you say its triggered something in him.  You need a good behaviourist to get him back on track.

we have not been able to let our boy off the lead if you do he gets real hyper his whole body gets ridged and he starts looking around for other dogs , he also was like this on the lead to start off after the incident but with lots of corrections he is no longer taking any notice what so ever on the lead

By "correcting" (punishing) him you have taught him that other dogs really ARE something to worry about as unpleasant things happen when he reacts to them. He's learnt he has no other choice but to "behave" when on a lead as otherwise the consequences will be unpleasant, but he hasn't learnt at all not to be worried about them -he's most likely scared. Your best option is to get help from a good behaviourist with qualifications  using reward based methods only, and start by showing the dog that he doesn't have to worry about the presence of others. You start this on the lead and as soon as he spots another dog, the moment BEFORE he reacts, you praise and reward with food, teaching him NICE things happen when others appear, not bad. Until he has learnt that on a lead, he's not going to be able to cope with the situation off lead.

Goldmali is right - I promise you he IS still taking notice, and probably a lot more than before because he knows he can't try to warn them off as he will be corrected for it, so will need to know where they are at all times.

What you have done is create a potentially very dangerous situation - a dog that's been corrected for giving warning signals is a dog that ends up in the papers, having "attacked with no warning whatsoever".

You need a good behaviourist - one who does not spout about dominance, corrections/punishment, use choke chains, spray/shock collars, water sprays etc etc, one that uses positive reinforcement and has experience with these sort of dogs because he's going to take more work than a lot of fear aggressive dogs - because first he needs to learn that it is safe to show his feelings again.  With a correction history that can be very difficult and take a lot of time - believe me, I've been there.  Your behaviourist needs to be good at reading body language - his signals will likely be very subtle now so they'll need to be able to spot them as soon as they are shown.

And just to back up what Goldmali and I have said - earlier this year I worked with a JRT, a rescue with bigtime fear aggression to other dogs.  He'd been being walked by a 'trainer'; her response was to correct any signs of aggression and like you, she pronounced him absolutely fine when he stopped showing them.  I walked him a couple of times and he was still just as bad with me.  I tried to explain this to her but no, because he didn't do it with her he was fine (actually he was seriously stressed but this 'trainer' cannot read dogs at all, if he was not reacting then he was happy to her).  So I had her ask his owners - lo and behold, he was worse than ever because he couldn't communicate with other dogs in the 'trainer's' presence and was being punished for it.  One day he'd seen a GSD - the breed he was most scared of - and bolted across roads etc.  So I worked with them, and he is now fine - through the use of distance, patience and rewards.