Whether a child care program would be beneficial really depends on your child's personality. At 19 months, many kids are ready for some time spent with peers. Some kids do better in a "Mommy and Me" situation than a child care situation without their parents. The best thing is to go and check it to see how he reacts and interacts. It may be a great thing for him, or it may be better to wait and try again when he is two.
Some of the behavior you are seeing, the observing rather than interacting, could be related to his developmental stage as well as his personality. All children develop in their own way and in their own time. The "observer play" you described is fairly common for children his age.
The down side is that even in the best, cleanest, child care situation, your son will be exposed to new illnesses and will get sick a lot at first. The good news is that studies are showing that this early exposure builds a strong immune system in the long run. It's just something to mentally prepare yourself for.
Go check it out, see what you think. When you see him in the environment, you'll have a better idea if now is the right time. The other thing is that if you enroll him and after a few weeks it isn't working out, you can take him out and try again later. Good luck with your decision.