How does one determine how much is enough on an injury settlement?

This is a very difficult question to answer.  We all wish we could accurately calculate a single number that would define fair compensation, but that does not exist.  There are several factors to consider in any personal injury matter in any state.

In my experience (in Ohio, Kentucky, and Florida), personal injury damages and settlements closely follow the plaintiff's actual injuries. So, the first place to look is at the injuries.  Your son missed school.  This usually does not result in monetary loss.  He has a few minor scars.  If they are truly minor, the value will be low.  Hospital bills total $6,000.  That is interesting, but what was actually paid on the bills?  Were they paid by insurance?  If so, watch out! The insurance company probably has a right to be repaid out of a settlement ("subrogation").  You missed a day of work - add that into the damages. 

It does not sound like your son has lasting or permanent injuries.  However, you should wait long enough to make sure he does not have these injuries.  Concussions can lead to mild or moderate traumatic brain injuries which have subtle but sometimes disabling symptoms.  Does he have changes in his behavior, emotions, thinking, schoolwork, etc.?  If so, you might want to consult a pediatric neurologist to make sure he will not have lasting effects from the concussion.  Mild traumatic brain injuries usually get better within 6 months or certainly a year.  If your son is having "mental" changes, you should delay settling the case.  If you think your son may have a serious and lasting injury, I recommend you consult a lawyer and do not attempt to settle this claim on your own.

There are some instances in which a defendant's really bad conduct will increase the amount of settlement, but usually it does not.  For example, a drunk driving defendant will usually pay more in damages than a usually careful, very concerned, well spoken defendant.  

Insurance companies usually pay higher settlements when a lawyer is involved.  However, lawyers are paid 1/3 to 40% of the settlement amount in most states (including Florida).  So, you would have to get 40% more in settlement to justify hiring a lawyer.  This is not usually a concern because many auto insurance companies refuse to pay even for the medical bills and force people to get lawyers.

The bottom line is that I can't give you a number that would be "enough" on an injury settlement.  Your brief description sounds like minor injuries that healed pretty quickly.  If so, recovering the medical bills, lost wages, and a little bit for pain and suffering would be reasonable. 

One additional consideration - many auto insurance companies these days are taking a hard line against paying substantial settlements.  They know that children heal quickly and, by the time your son's case could get to trial, he is likely to be just fine and a jury would award very little.  They also know that many jury members think people who sue are greedy and won't award much in damages.  If you consider the hundreds of thousands of auto negligence cases, insurance companies are going to pay substantial damages on only a few.  Therefore, it pays for them to hold out.  Unfortunately, this is what you are up against.

I've probably said more than necessary about this, and I hope it helps you decide.