20 month American Staffy....?

I own a 20month American staffy, Boy. He has a beautiful nature and is just a big softy really.!! Loves all people and gets on great with any dog. He is very excitable, but he still young.! when on walks, if he spots another person and/or dog he becomes fixated on them and as they walk past he lunges at them. Not is an angry way at all, just wants to say hi..... any ideas on how to calm him down when we see others on our walk.? (watch cesar milan so know to stay calm etc.)