It's difficult for some young children to play with others, especially if the child is worried about having to share his favorite toys with a stranger. Being away from home can make relating to another child even more stressful. While much of this behavior will go away with time, there are a few things you can do to speed up the process.
First, find one playmate whose temperament is compatible with your child's style of play. By having repeated play dates with one child, your son is more likely to be comfortable interacting with another toddler.
Second, keep their time together brief, especially in the beginning. Start with only a minute or two and then add to the time as they build a relationship.
Third, don't expect your son and another child the same age to play on their own, especially at first. You should take an active role, which will help your son feel more secure and be more willing to take risks.