D.T.
I notice my daughter switches back and forth between being dominate and submissive. With her it's simply that she's still learning how to interact with others. She's an only child at the moment so she doesn't have a whole lot opportunity to play with children her own age.
This could be similar to your son's situation. He's still learning how to behave. If you are with other mothers and their child is rough with yours, you can try gently comment to the mother. It could be something he grows out of as he learns, it's hard to say.
Good luck.