SofiaAmes, unless your husband is interested in the kind of scientific/mathematical subjects where you have to acquire information step-by-step in order to progress, it would really be better for him to give O-levels and A-levels a miss, and look into Access to Higher Education courses. As Suedonim says, these are usually run by FE colleges, often with a direct link to a local university, and they are designed specifically for people in his situation. Unlike O and A levels, they take into account the life experiences he's had, the fact that mature students are usually very highly motivated and committed etc.
I think I've picked up from other posts by you that you live in London, so there will be lots of courses around. A lot of the London universities really focus on mature students and those who come into HE via non-traditional routes, and some have access advisors - it might be worth ringing them to ask for advice, as they might be able to recommend the best local access courses. Try University of East London, South Bank University, Middlesex, U of North London, for starters.
I wish your husband the very best with his efforts to get an education. As a university lecturer, I love working with students like him, they bring so much richness and diversity to the classroom and are so committed to studying.