Does he have a realistic idea of what being a SAHD entails, so how much time for studying he would have during the day?
Why would he not be able to do an OU course? Does he work extremely long hours? They are designed to be done part time while working and that's how most people do them. If he couldn't manage his time to fit that in, I don't really see how he'd combine a full-time course with being a SAHD.
An advantage of the OU would be there are no entry requirements, though there are introductory modules for people who haven't studied before / for a long time. Another advantage is low cost compared to other universities, though you still have to know you are able to commit to studying, week in week out for nine months, to pay over £2,000 per module (though there are bursaries if you haven't previously studied for a degree).
It sounds as though you have established a habit of studying and built up your knowledge and CV, so are in a good position to know what the relevant degree would entail and whether you can cope with it.
He comes across as jealous, seeing you as having had the easy option, being able to make choices and pursue interests while he slogs away in a boring job. He may have a point there but, only if he really has the measure of the SAHD role and, he could have pursued his interests in his spare time with an A-level or access course and has chosen not to. It sounds as though he is a dreamer, taking out his frustration with his own lack of organisation, motivation and ability to realise his desires on you (just as his 'time out' from the family was the pursuit of an unsustainable dream).
I really sympathise with your desire to be able to support your family with a better paid job. However, to get there as a family, not making your worst fears of being left alone come true on the way, I think you are going to have to give something, for him to agree to your studying full time for your course. Can you work for a year, or part time? Or plan to support him when you've finished so that he can do his entry qualifications in the meantime, maybe get started part-time or be ready to go with a better idea of what he's studying and why when you've finished?
I really think though, if it's a hobby degree (and let's be realistic that degrees per se do not guarantee improved employment prospects any more) he needs to do it in his spare time. If he started gently with the OU, then by the time you're working full time, he might be able to take a year off and do two modules, so complete his course in a full time manner - and by then the children will all be at school, making that more reaslistic.