My 9 year old is 52 lbs. He's been in a low back booster for years. Seems like it's fine, according to the new law.
Here is the new Maine law:
2-B. Children 2 years of age or older and weighing less than 55 pounds. When a child who is 2 years of age or older and who weighs less than 55 pounds is being transported in a motor vehicle that is required by the United States Department of Transportation to be equipped with seat belts, the operator shall ensure that the child is properly secured in a child restraint system with an internal harness in accordance with the child restraint system manufacturer's instructions and the vehicle manufacturer's instructions except that, if the child exceeds the child restraint system manufacturer's recommended height limit for the child restraint system, the operator shall ensure that the child is properly secured in a federally approved belt positioning seat. Violation of this subsection is a traffic infraction for which a fine of $50 for the first offense, $125 for the 2nd offense and $250 for the 3rd and subsequent offenses must be imposed. A fine imposed under this subsection may not be suspended by the court.