Aggression on the lead

Since we got Murphy, Max has started to be quite aggressive when he is on his lead (not all the time).

For instance, we (Max, Murphy and I) met another Boxer on our walk (locally, on lead walk) and Max didn't take to him at all - he growled and lunged at him.

But a few days later, we met the same dog off lead and Max was fine with him.

At training (without Murphy, on lead) there are a couple of dogs who Max has taken offence at, and he growls and lunges at them; but when we have met them off lead on a walk (organised by the traning club) he has been fine with them.

I can't work out whether he is generally feeling very protective of Murphy and I when on lead, so feels the need to show aggression, or it is another sort of aggression.

I'm also not sure how to deal with it to be honest. At training, I would place myself between Max and the other dog, break Max's eye contact and then remove us from the situation.

Is this what I should be doing when we're on a walk as well?

It sounds a bit similar to the problem i have when walking lady skye and beau together , lady feels as though she has to protect the two younger ones , i always distract lady and break her eye contact with the other dog , and tell her to leave , it works ok for me but im sure some one elsehere with more experience can offer better advice xxxdk