A genuine change of residence is one of the few cases where VRT is exempt. However, you need to tread very carefully, as there is a limited time a UK resident can use a car here, and it is definitely not OK for an Irish resident to use a UK registered car. I can't see anything wrong with what you suggest if there is a definite plan to move back over a period of a few months, but if it's just a vague plan to move back at some future time, he would probably run foul of the requirement to import the car after a certain period.