One of the most important challenges faced by toddlers is learning to handle social relationships outside of their families. Both your son and the children who are taking the toys are struggling with the same issues. Instead of focusing on assertiveness, I suggest looking for ways to give him practice with important underlying social skills.
For example, when my son was that age and a bit older we'd play a game we called "cars." We'd divide his collection of miniature cars into two sets: one for him and one for me. Then we'd start trading. "I'll give you the yellow Ferrari if you'll give me the truck." When we struck a deal, we'd each send our cars skidding across the floor to the other person. He loved it and would often play for an hour or more. More important, it gave him some experience with negotiation figuring out how to get what he wanted by developing empathy and listening to another person's needs.
Be patient. Talk to the teachers at the child care center for some perspective. Find out if they're concerned. (I doubt it.) Remember that it's important to give your child enough time to learn and master these important social skills.