The good news is that nose picking is usually a symptom of boredom or curiosity, and is more of a stress relief mechanism for your child than a sign of nervousness or anything else. It may, however, be an early warning of an allergy, as children with allergies tend to produce more mucous. If you are worried about allergies, your doctor can perform a test.
Drawing attention to the behaviour by admonishing your child can do more harm than good, because you make him aware of it. Most children will outgrow this habit by themselves, when they reach school, and their friends and peers start to tease them about it.
In the mean time, keep his hands busy with other things, and hopefully, he won’t have the time or the inclination to pick his nose!