Rotweiller Dominant over other dogs

Question
I have a three year old Rottweiler who is submissive at home, through extensive training.  When at the off leash park, he is too dominant over other large breads.  He is fine with small breads.  His behaviour is placing his head over the other dogs back or bumping the other dog with his chest.  He is also fine with our other Rottweiler that I have instilled as the dominant one, at home.  How do I curb this behaviour at the off leash park?

Answer
My first question is "Is he intact?".

Young Intact Males often show this kin of behaviour, and at 3 he could still be classed as a young or juvenile rotti.

If not, what I really need to know is at what level on a scale of 1-10 is he pursuing this.  Does he show signs of getting aggressive,has he escalated into growling or biting, how in general to the other dogs react, what is his body language?

The reason I ask is that some dogs just do play ruff.
Dogs speak Dog better than we ever can, so watching the reaction of the other dog, gives you a clue as to wether this is ruff play or true dominance.

As with all unwanted behaviour, a quick correction by handler, will let the dog know, that this behaviour is unacceptable in their presence.  Use of a leash even in off leash parks is acceptable to control the level of play, excitement.  Another distance alternative is a Water Pistol or vibration collar. (never a shock, as the dog wearing it my react to it as a bite from the dog they are playing with, plus I do not belive in causing pain to train).

The trip to the park and the way we enter is also important.  Your dog must be clam and relaxed before entering and you want to see that other dogs are also in a clam relaxed mood and that there are no overly excited dogs about.  Over excitement and stimulation can quickly turn to aggression.

Training your dog to respond immediately in high distraction environments will help, to call him away, when he gets in a bullying mode.