You should not be worried as there are always kids who do not know all the colors. It is developmental and the best thing you can do is continue to teach your child as repetition is important. If you are concerned about your child’s sight you could take him to the optometrist.
Optometrist have simple tests for color blindness. One way to check this problem is to make him match things by color. Take a pile of sorting bears that are all the same size and shape but different colors. Then make him put the red bears in one pile, blue in another and so on. Matching is usually a skill that develops prior to being able to identify. If he can’t match after a little practice then there might be a color blind issue.
While doing anything with your child mention colors. When you pour milk say something like “lets get your BLUE cup and pour you some milk” or “Put on your WHITE socks.” Do not bother with the books and computer. The best way to teach your son colors is to actively show him and say what the colors are. A good way to teach colors is to read an easy book and point out specific colors.