Your husband can keep his South African nationality if before he naturalizes in Britain he applies for and receives permission to do so:
Application for the retention of RSA citizenship is an application that MUST be made before one acquires foreign nationality. South African citizens who take up foreign citizenship without applying for retention of South African will lose their SA citizenship. Persons under the age of 18 years do not need to apply for retention of their South African Citizenship, as they will not lose their citizenship whilst still minors
A person who has lost citizenship by virtue of failure to apply for the retention of RSA citizenship will have right to permanent residency in South Africa – if he or she was born in RSA. Such a person will be allowed to apply for the resumption or reinstatement of South African in South Africa. An application for resumption or reinstatement of RSA citizenship is approved on condition that such an applicant will live permanently in RSA once his or her application has been approved.
The UK has no special requirements related to dual citizenship:
Dual citizenship (also known as dual nationality) is allowed in the UK. This means you can be a British citizen and also a citizen of other countries.
The process is expensive. Your husband has to acquire indefinite leave to remain or to enter, and to meet a three-year residency requirement. His time in the British military may or may not count, depending on when he makes the application and whether he lived in the UK when he served.