"I've gotta ask a really silly question, what should he do to fix what he's done - because I honestly can't think of a thing!"

I agree with what the others said above. Also something else a friend of mine did when this happened in her relationship was she told her DH he had to 'woo' her again.
Basically show her she was his only one, take her out on dates, Buy her special things, let her know he cares about her. Think about it as 'starting from scratch' again. If he wants time from you then he needs to organise time to be with you.

Hugs, I hope things work out for you.