Utah and Idaho both take part in the Drivers License Compact (DLC) which requires member states to report moving violation convictions received by motorists back to the state where they are licensed. So if he pays the ticket, and thus pleads guilty to the speeding offense, Idaho courts will notify the Utah Division of Motor Vehicles of this out of state speeding violation.
The UT driver's handbook notes this about the DLC - The DLC provides guidelines for greater cooperation among members in driver license matters. Among other things, member jurisdictions exchange driver records and forward convictions for traffic offenses to the driver's home state.
The Driver License Division of Utah maintains a computerized driving and accident record of every driver in the state. It contains a record of convictions for moving traffic violations and suspensions within the past three (3) years. All alcohol/drug related violations, suspensions, and revocations remain on the record for ten (10) years.
When a driver is convicted of a moving traffic violation, notice of the court action is sent to the Division for posting on the driver's record. Also, if you are a commercial driver, your driving record will be submitted to the Commercial Driver License Information System and a record of suspensions and revocations will be forwarded to the National Driver Register.
The handbook does not state for certain if out of state violations are placed on a driver's record or not but UT DMV site states that Utah is part of a compacts involving all 50 states and Canadian provinces, which shares information regarding convictions for traffic violations.
Therefore, conviction of specific offenses causes a record of conviction to be sent to the Utah Drivers License Division within ten days of the conviction or bail forfeiture. For example, convictions for speeding offenses will be reported to the Drivers License Division. However, some non-moving violations, such as lack of registration, will not be reported.
You will need to contact the UT DMV though to find out for certain if your son's ID speeding ticket for 13 mph over the limit will be assessed points in Utah or only placed on his driving record. Once it is on his driving record his insurance company will be able to see it the next time they pulled his MVR. It will then depend upon the insurance company's rating system to determine if this speeding violation will cause his rates to rise or not.