You may find as said before that its a case of him being scared and while you have him on a lead he has no freedom to run away from what he sees as a potential threat to either him or you . This can be a problem in as much as a normally friendly dog can seem to be agressive to someone who doesnt know him , or why he is barking .The lead has an amazing affect on a dog not used to being on one, a dog has 3 choices when confrunted with fear , Run away, Freeze , or Fight when you have him on a lead you have taken away his first choice of run away , because if he was loose and met a dog in the park he would approch on his tearms in his time and they check out each others body language too weighing up if they are friend or foe .Maybe it could just be a case of perservering keeping a strong front that you are boss and if you think the other dog is cool then he should adopt the same thought
as leader your judgement is paramount and sometimes if they question your leadership they will start to make up thier own minds about what they do and dont like .
Andrea

__________________