when a child turns 18 can he sue for a car accident when his mom settled for him

This would depend on what your mom signed when she settled your injury claim.  Typically, however, an adult has the authority to compromise a claim for a dependent minor.  Furthermore, an insurance company would not likely settle the claim without getting a full release from the parent or guardian, which would include an acknowledgement that by accepting money, the parent was releasing all future claims of that child.  If the amount of the settlement exceeded $7500.00, then the probate court in the county where you resided would have to approve the settlement as well.