<human>: My SO and I[F] rescued our adult, 55lb shar pei/boxer about 1.5 years ago.  He has overcome many obstacles (heartworms, separation anxiety..) but has always been a very anxious dog.  I think he had an owner that trained him very well but then something happened and he ended up as a stray that has had a hard life and has the scars to prove it. I don't think he had much or any socialization with other dogs because he would do the lunge-and-attack quickly followed by a retreat, usually with him standing between my legs shaking whenever he saw another dog.  He has gotten MUCH better with his interactions with other dogs-no longer lunging, starting to meet and greet some other dogs and even behaved like a gentleman when he was surrounded by a pack of small dogs.  

I'm so glad he has overcome all of these but now he is starting to air-bite or nip at men.  It has happened a couple times now, once our gardener came into the backyard unexpectedly and he air-nipped him.  He alternated trying to be petted with growling and air nipping at my sis' ex. (He was talking with his hands at the dog's level and he tries to pet himself with your hand if he can reach).  Our landlord came over to work on something in the garden and our dog didn't act out when we weren't there but nipped at him when I was home. And most recently, my BF took him into his work office and when a colleague came into his office he bit him resulting in the bruise.  We have tried to disciple him as best as possible-he has been in obedience training and I have downed him and told him, "no." when I was there, my bf told him no when he was in his office.  These instances have all been at least 2 months apart.  Any suggestions? I just don't want this to get out of hand.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: Adult rescue dog has started nipping and air biting men in the past few months.  How do I stop this behavior when it happens infrequently and never was an issue before?