I understand your concern - you don't want your child to become a victim later on. The good news is that he doesn't need to be aggressive in order to avoid that - he can be ASSERTIVE. There are three stances a person might take in the face of someone else's unacceptable behavior: passive, assertive or aggressive. The first one involves giving up, doing nothing, allowing oneself to be mistreated. The third one involves equally unacceptable behavior or worse. The middle stance - assertiveness - is the ability to firmly but respectfully set boundaries. In toddlers this might take the form of using words rather than hands. For instance, if a child grabs your son's toy, he can say "Don't do that" or "That's my toy" or "Teacher, he took my toy." At this point in time, your son doesn't care enough to assert himself and for now, that's fine. However, I think you should take some books out of your local library on the subject of bullying. Read the stories, show your son the pictures and talk to him about his experiences and his options. You can help prepare him for life ahead - dealing with issues that matter more to him in the future. He just needs to know that he has choices and that he is allowed to defend himself or correct the behavior of others. There is no emergency; take your time and work on this issue with him slowly. Use some role playing as well, or maybe puppet shows. There are some story books for toddlers about this subject as well - there was one put out quite awhile ago from Sesame Street about Grover's lunch box (kids kept asking him for his sandwich and his snacks and he gave them all away because he didn't know what else to do....). Perhaps you can find that or ask your librarian for some help.  Good luck! Sarah Chana