His only legal entitlement, under the Working Hours Regulations, is to a single 20 minute (unpaid) break. (He becomes eligible for a 20 minute break as soon as his shift goes over 6 hours. However the entitlement is neither 'cumulative' nor 'pro rata'. He'd still only be entitled to a single 20 minute break if he worked a 24 hour shift).
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10029451
However, Health and Safety considerations might also apply. The relevant legislation doesn't define any specific break entitlements but it does require that employers should ensure that their employees' health and safety is not put at risk through their working practices. If, for example, your boyfriend was suffering back pain through continuously sitting at a desk (or from through repeated lifting operations) his employer would have to adjust his working pattern to alleviate the health risk. However, the employer would still not be obliged to provide any additional breaks; he could simply move your boyfriend to a different type of work.
Chris