The actual (paid) holiday entitlement is 22.4 (days per year. (An employer can round it up to 23 but isn't obliged to).
However the boss seems to have already broken the law anyway, as he must ensure that all employees are provided with a 'written statement of employment particulars' (which isn't quite the same as a 'contract', per se, but must include information about holiday entitlements):
https://www.gov.uk/employment-contracts-and-conditions/written-statement-of-employment-particulars
Any contract which contains a term which contradicts the law is automatically invalid in respect of that clause (and possibly as a whole). So, irrespective of whether your husband signed such a contract or not, he'd still remain entitled to 22.4 days paid holiday per year.
The first place to start is probably the Acas helpline:
http://www.acas.org.uk/?articleid=3282
(AB members have reported in the past that it can take a long time to get through but that it's worth the wait).