Hiya,
It's great that you have a reliable cue to use to get him off when he has already jumped up but what you obviously want is to change the jumping up in the first place. I would work on teaching him an alternative behaviour to greet people, such as a sit/stand. Make him sitting or having all four feet on the floor be the thing that determines whether he gets to say hello to the person he has his sights set on. He will understand it more quickly if the timing is good, so if does as you sk then he should be immediately rewarded with the hello he is after especially at first. If he tries to jump up the person should immediately move back out of his reach for several seconds and then try again (ask the person not to focus on him look at him etc) as this will only likely make him more excited and frustrated and less able to control himself at a second attempt.
Explain to people what you want them to do and be strict with them because it's not fair to expect him to understand if everyone doesn't do the same thing. This approach really does need consistency especailly at the beginning. It can be really difficult when you are trying to train a dog not to jump up and you get people saying, ooh I don't mind while they are fussing the dog and being all squeaky while he is in the process of doing exactly what you are trying to avoid.
Do it every time for greeting at home, if you have visitors coming over and he does it with them then pop him in another room while you let them in and explain what you are doing and then bring him into the room on a lead to start with if needed. Until he has the hang of things with people he knows and consistently at home distract out on walks as you have been doing, maybe use something more smelly than chicken, maybe garlic sausage or liver and avoid just using it as lure, reward him with small pieces especially to start with or as someone else suggested use a special toy to get his attnetion.
When you start feeling that he is getting the idea that all four feet have to be on the floor to be able to say hello then you could start explaining ot others that may want to say hello to him, you'll just have to remain in control of the situation as strangers won't always follow what you ask.
It may also be an idea to give him an outlet for allowed bouncing, he is a boxer after all

After my Dobergirl got the hang of greeting nicely I taught her a jump up on cue so that she could get her fix of boinging!