Rambunctious BIG baby. What can I do?

I have an English Mastiff who is the same age. We have a command called"sit and greet" and we use it when he sees anyone he wants to say hello to. We are strict and if he doesn't sit in front of the person, we keep walking or move to another room. He now will sit in front of people and look at them, then us, then them sitting still but tail wagging wildly. Please don't forget that for this breed, meeting people or getting to greet children is a treat for them and you should ever give that reward without them doing something to get it. Its a marvelous breed so enjoy and good luck!