Has anybody got advice on how to stop my dog barking at all other dogs?

I adopted a JRT from the dog shelter almost 3 months ago. He is about 7 years old with an unknown history. In the house he is quite good after initially being a bit too possessive over me i.e growling at my partner. But outside can be a real problem. He goes absolutely crazy when he knows it's walk time. Shaking and whimpering. I have tried waiting until he calms down, but he doesn't. It just turns into a howl. Since buying a control harness he has also reduced the pulling and is nice to walk until we see another dog. No matter how far away he starts barking and this will continue long after the dog is out of sight. He is not really trying to pull towards the dog all the time and usually he is in play position. He is oblivious to any treats (even his favourites) when we are out. The few times that somebody has let their dog come over, he appears that he wants to play and then jumps on their head and upsets them. There has been some snarling and scuffles but I am not sure who started it. He even pushed a spaniel over and stood on it's stomach. The owner was trying to help by letting them play but Max does not play nice. Now nobody comes near us. A couple of times we have suddenly come face to face with a dog and if it's an old uninterested dog he is fine. I can't let Max off the lead as he has run away from me twice which has caused immense stress and upset. I am a loss what to do as I think he needs to socialise bit it has turned into an incident almost every time I have let him near a dog. We got put in a separate waiting room at the vets and he was being aggressive to all the dogs that came near us. I have tried the water pistol and I have tried putting him on his back to calm down. Neither work although once the dog is out of sight he will often roll over onto his back and look up at me. I am prepared to get some help but it's expensive so am willing to try anything myself first. Thanks for reading,