I don't think it is a huge issue at the moment. As you said, he has only just started understanding about playing with others. Once he gets used to this, he will probably get more assertive.

My DS1 was very much the same. He has also just turned 2. A child would take the toys he was playing with and he wouldn't do anything except call out to me. A couple of weeks ago that changed and now he hangs on to the toy and yells out "MINE!". He even growled at a little boy who tried to take his car. The other boy dropped the car and took off. I couldn't help laughing at him.