Your boyfriend has no legal right to receive the same pay as other workers. Unless the employment is covered by national legislation (such as teachers) or by collective bargaining arrangements (through a union or staff association), each employee's contract is entirely separate from that of other employees. As long as the employer pays the National Minimum Wage, and doesn't discriminate in ways that break the law (e.g. on the grounds of race or sex), it's entirely lawful to have employees doing similar jobs paid at different rates.
However, your boyfriend still has the right to raise a formal grievance, simply on the ground that promises have been broken. For advice about how to do this, start here:
http://www.direct.gov.uk/en/Employment/Employe es/ResolvingWorkplaceDisputes/DG_10027992
Chris