There is no hard-and-fast rule. First, what are his current medical expenses? That forms the baseline. Related to that, is it anticipated that he will need future medical expenses and are they reasonable provable or certain? If so, add to the current expenses.

If the "buckling of the knee" is likely to represent a permanet impairment, then your son may be eligible for additional compensation for pain and suffering. A very rough rule of thumb is that 2 - 4 times medical costs is reasonable.

So, if current medical plus reasonably certain future medical exceeds $1,500, and/or if you son would reasonably be able to sue for pain and suffering which, with medical, will result in a claim for more than $1,500, it may be worthwhile to consult a lawyer and push for more, though you need to factor in legal fees of roughly 1/3 the claim amount and also a few hundred dollars (at least) of expenses--more if you need expert witnesses.

Medical expenses are separate from any claim for pain & suffering.

Medical expenses are covered under the no-fault insurance of the car that hit your son.

The  questions that need to be answered are:

    Where is the scar?

    Is it permanent?

    As to the knee, has you son been seen by an orthopaedist? Why is the knee buckling?

Whatever they are offering does not and should not include medical expenses.

You shoud speak with an attorney. The consultation shold be free.

Good lluck,

    AJL