R.S.
I could be wrong, but I think he just loves all the attention he gets from you....your heighthened reaction when he acts this way. Kids want/desire attention so badly they will take negative attention just as easily as positive.
I would go out of my way to give him regular one on one time everyday (15-20 min set a timer) and praise him when he is nice to anyone of any age. I am sure you will get some great ideas from others on discouraging the negative behavior....I would immediately leave any situation when he acts that way (warn him before going to the park etc. what will happen, and when it happens do not even blink, say "we are going now because your behavior is out of control so I am going to help you control yourself be taking you home..." he will tantrum the whole way home and then some, but you will probably only have to do this once or twice. After he calms himself down tell him you love him, but that his behavior of hitting/growling at young children is not right and that you are going to help him control himself by taking him from those situations until he figures out another way to act.
Also watch him carefully he is probably reacting with growling vs. saying "I have this toy, you can have it in a minute...." etc. so work on giving him the words he needs to negotiate in the social realm.
HAng in there this seems like a phase.