Children with allergies are likely to pick their noses because they have excess mucus that makes their nose uncomfortable. In this case, using a humidifier or saline nose spray can help.
Unfortunately, there is not much a parent can do to change this behaviour. In most cases, if it is a nervous habit, your child will give it up eventually.
All you can do in the mean time is keep reminding your child not to pick his nose and try to keep his hands occupied with other things. For instance, if you notice this behaviour when your child is watching television, he probably is unaware that he is even doing it. Divert his attention by giving him a rubber ball to squeeze.
Of course, if your child picks his nose to the point where he is causing it to bleed you need to watch it more carefully. He may have additional nervous behaviours as well such as thumb sucking or trouble sleeping. In this case you’ll want to take him to see his paediatrician.