help barking and lunging on lead

my 2 year old staffy x has began barking and lunging on lead, he was very well socialised as a puppy and met lots of different sized and different aged dogs due to the fact he loves to run and run i walk both dogs in a enclosed field,unfortunately very few people walk there.he has begun barking and lunging whilst on lead but with no aggression(at least i don't think so)it appears more to be a warning type of action, i believe the thing that set him off was a labrador having a pop whilst he was onlead, first of all it only appeared to be dark dogs that bothered him but now it is all dogs.if i have my other dog with me i let her go to the other dog and he calms right down and is happy to say hello i think he must be frightened i have tried to treat him when he behaves well but he isn't really interested in food i am going to the vets tonight so will pick up a leaflet about local training classes but i find it difficult to get to classesas i am disabled.
i should also point out that when i walk him i have a crutch in my left hand(sometimes both hands) and the dog/s in my right so no spare hand.he is also neutered and in good health(had his vet check last night)any suggestions gratefully recieved

I would try a halti or dogmatic takes a while to get them use to them but they do work,my dog walks really well until he sees another dog then nearly pulls me over he likes to say hello to every dog he sees.

thanks lynn had my other dog to the vet this evening and spoke to him about my concerns and he is going to help me out hopefully i can get him back to how he was and get him to realise that just because a dog has a pop at him not all dogs are bad as if i would let anything happen to him