I'd second Run, has he asked if he can go back to his pre-promotion job?
Otherwise, presuming that he started work one year ago for this employer and his previous work was with a different employer, he doesn't have many options. You can be dismissed for any reason within two years, providing it's not discriminatory, which this doesn't sound. There's no statutory redundancy pay before two years, either.
I'd polish up his CV and start applying for suitable jobs. If he's got friends in similar work, ask them if they know about any opportunities, too.
Do you have any children?
He'll probably be entitled to contributions based JSA, which is £57.90 between 18 and 24 and £73.10 if he's over 25. I believe this lasts for 6 months, and then he'd need to be assessed for income based JSA, which would take your income into account - if you earn over £6000 a year, his benefit will be reduced accordingly.
Could you increase your hours at work?