It is probably just a variant on 'worth his salt' or 'worth his weight in gold' and similar phrases, all implying 'of great value'. I've no idea as to its origin and even Google appears never to have heard of it.
I once heard that it's possibly nautical i.e. sailors were often given ships' biscuit as part of their rations, and these often looked like crackers.
It therefore implies that the man is well worth his rations, or has earned them for his hard work.
Colleagues of mine in the Army were often referred to as 'a waste of rations' i.e. for what they did they did not warrant their food.