Secondly, achieving these developmental milestones earlier in the range does not in any way indicate that a child is smarter, and achieving them later does not mean that the child is delayed. Each child will develop at his or her own pace, and while it is important to encourage and stimulate children as they try to master each developmental skill, I would not place too much importance on trying to achieve them earlier than other children. Keep in mind that there are different areas of child development, and a child who has early language skills may not be as early with his motor skills, and vice versa. If a child is actually delayed in development (that is, below the normal range), then that should be addressed as soon as possible with the pediatrician.
It sounds as though you have concerns specifically about your son's speech and language. You are doing the right things by making sure you speak clearly, and playing games where you encourage him to repeat or sing songs. It is also important to make sure that your son is hearing adequately. You should talk with your pediatrician further about this. If your son is truly delayed in his language development, then the first step should be to have his hearing tested.