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Deferring for One Year
He should contact the universities he is interested in attending and ask if they will allow him to change the offer to 2019 entry. If it is for relevant experience I don't see why they would object, but it is their choice and they must decide. UCAS will then be updated.
I think it would be best to do this now, rather than at results time (as long as he is certain he wants to change his mind).