Does he bark and growl when in the presence of a fellow canine? There are plenty of ways to help your small dog become more of a social butterfly.
Often the way your little one reacts to humans is based on past experience. For instance, if he was handled roughly at any point, or even just picked up in a way that was uncomfortable, this can cause him to associate people with a memory of discomfort. Additionally, since children tend to get overzealous when approaching or playing with dogs, you should take the initiative in supervising interaction between them. Don't be afraid to tell anyone the way you prefer your little dog be held, played with, etc. Your dog is your responsibility and relies on you to make sure that no one, even unwittingly, does something that will upset him. Ultimately, by ensuring that your baby is handled in the best manner possible at all times, you will be helping him to enjoy the company of friends and strangers alike through trust.
When it comes to socializing with other dogs, play dates are an excellent, controlled atmosphere where you can gradually introduce new furry friends to your little one in an environment that he may already be comfortable in. When possible, invite dogs of different sizes to the play date (of course, first speak with the "parent" and make sure their furkids do not have aggression issues), so that your little one learns that there's nothing scary about dogs that are larger than he is. Be conscious of things like toys and treats; make sure toys are shared and that treats are given to every dog in attendance. Join in a bit, and play together with your dog and another dog to show your baby that interacting is okay and fun!
An outdoor walk is also a great way to socialize! Use a high-quality dog harness to make it comfortable. Or, get a doggie stroller - always a conversation piece, which, in turn, draws lots of people over to say hello.