This will not be the first time that he will hit his head that you hear the crack. I have a pretty active dd and she has had her fair share of falling out the cot, bumping her head in the bath and just plain falling down in the run. It's not nessesary to keep him from his sleep. The thing you need to look out for in a concussion is if he vomits or looks disorrientated.

Here you go hun ... got this off the net for you

"What's a concussion?
When a closed-head injury — meaning one in which no object penetrates the skull — causes a change in the normal functioning of the brain, it's called a concussion. The injury might be from a violent blow, a fall, or a severe shaking.

A child with a concussion may lose consciousness or have problems with his vision, memory, or balance. This sounds scary, but in most cases the effects are minor and temporary and the child recovers completely.

How can I tell if my toddler has a concussion?
Look for one or more of these symptoms following any trauma to the head:

• loss of consciousness

• headache

• dizziness

• confusion

• nausea

• vomiting

• irritability

• drainage, clear or bloody, from his nose, mouth, or ears

When should I call 911?
If your toddler hits his head and starts breathing irregularly, has convulsions, or is unconscious, call for immediate help. Don't move him unless he's in danger of being hurt further. Perform CPR if he isn't breathing, and if he's bleeding, cover the wound with a clean cloth and apply pressure.

When should I call the doctor?
It's common for children to hit their head when they fall, and in most cases there's nothing to worry about. If your toddler loses consciousness, though, have him checked out by his doctor or by the doctor at the emergency room.

Sometimes, even with a minor blow, the brain can be injured. Also take your child to see a doctor right away if he hit his head and in the next day or two he:

• Has a lingering headache, lightheadedness, or vomiting. It's okay to have a headache that's relieved by acetaminophen or a bout of vomiting after a fall. But neither should persist.

• Seems unusually sleepy during the day or can't be awakened at night. Try waking your toddler a couple of times the first night after his fall, just to make sure you can.

• Has any problems with his coordination, mental ability, or strength — like weakness in his arms or legs, slurred speech, confusion, or impaired vision.

Can a concussion cause permanent brain damage?
Only in rare cases. But a second concussion before symptoms from the first one have cleared can be dangerous — there's a higher risk of brain damage and even death. So if your toddler did have a concussion and lost consciousness, your doctor will probably advise you to keep him from getting too rambunctious for a couple of weeks."

With what you described .. I would not worry