Nose bleeds are quite common in children.
The most common reason is nosepicking.
The front part of the nose contains lot of blood vessels and nose picking results in bleeding.
Also when the nose is dry and the dry layers gets removed it results in bleeding.
You did the right thing by applying vaseline over the dry area.
Applying pressure over the nostril for sometime will arrest the bleeding.
Coming to your apprehension about leukemia, it is unlikely as the CBC was normal.
If it was leukemia, the CBC would have shown abnormal counts and low hemoglobin.
Since it was normal, I do not think he has leukemia.
All you have to do to prevent bleeding is to make sure he does not pick his nose and also apply vaseline if his nose is dry.
I hope that answers your query.
Let me know if you have any more questions.