Hello and welcome to JustAnswers,
Common question on our site as the Rimowa often will lock for various reasons.
As for ways to open it, try having your son review this overview on how to reset the lock:
If none of these suggestions work, then what will be needed is to contact a qualified repair person who deals with this type of case. Only qualified repair persons are those that can help in getting the case opened. Your son can check with the site below on the closest repair shop:
I hope that the information provided is found helpful as please let me know of any additional questions/concerns. If you could please take a moment to rate my assistance so that I can receive credit for helping you this evening.