Here's a (semi-) analogous situation, which was reported in the press. (The analogy isn't perfect but it provides a starting point for considering your question):
A guy ran a one-man rural taxi business. His car broke down when he was due to provide transport for a regular customer. As an act of goodwill, his wife collected the customer, in her own car, to take him to his destination. She told him that, because she wasn't licensed as a taxi driver, there would be no fare charged. She was involved in a minor traffic incident and a court ruled that she had no valid insurance because the journey was 'for hire or reward'. This was despite the fact that the lift was given free of charge. (The court took that view that 'maintaining goodwill' constituted 'reward').
So it's clear that the absence of payment doesn't prevent a court from taking the view that someone is 'working'. Anyway, in your husband's case, a court might take the view that he was 'paid' because he benefits from your income which is obtained through running your business.
I can understand that your husband would want to help you with your business but I think that DWP investigators might take the view that your husband is doing undeclared work.
Chris