My 5 year old son bleeds from his nose from time to time, what should I do?

See an ENT doctor. It is a common condition in children and is due to prominent blood vessels located close to the surface on the front part of nasal septum. Depending on the child's cooperation, it can be cauterized under topical anesthetic or under general anesthesia with high success rate!
Minor treatment. Humidify his room at night. Put something like vaseline in his nose 3-4 times per day for 4-5 days after the bleeding. Nosebleeds usually are from dryness, allergies or nosepicking. Make sure he is not bruising easily, bleeding while brushing teeth, or an " easy" bleeder.If these occur, see your pediatrician asap.
Nosebleed. In children, place a dab of vaseline in the nostrils and squeeze the nose. This will act to apply the vaseline to the septum. Usually bleeding comes from the front (anterior ) septum. Encourage no picking of nose. If moisture doesnt help, then see ENT as sometimes cauterization is needed. There are also minor nose bleed control kits which can be purchased at the drug store.
Vaseline. See a ENT DR. Meanwhile apply vaseline or Aquaphor inside both nostrils. A tiny dab. Gently rub around. If there are no more bleeds you can cancel the ENT. This will usually stop it.