Hi, OP -
I think this is a bit subtle. Ad his parents, you do want to help set DS up for the future.
It’s fantastic that he has this passion for Theology. I am normally one to suggest that students follow their passion, but the reality is that the success stories above depend on transferrable skills and personality traits — flexibility, verbal fluency and multitasking chief amongst them — that may or may not possess, or wish to develop.
I agree with @titchy that if he envisages taking a job in science a relevant degree will be very useful. I also wonder whether he has realised how much writing he will need to do for a Theology degree? Amongst my autistic personal tutees (I am in STEM) essay based elective modules have sometimes been a source of great anxiety as the assignments and exams are inherently less structured than ours. And my students are just doing one such module at a time!
Does DS have the qualifications to apply for Theology? I almost hope not, because the need to do an essay based A level or two would be a first sample of whether the Theology degree is right for him. If he has already done this, of course that’s a big first step taken.
If upon reflection and perhaps further effort DS remains committed to Theology, I would hate to discourage him completely. I don’t know what type of university he is aiming for, but ideally I would suggest a Joint Honours degree. At Exeter he can design his own. At Glasgow, one of the great hidden gems of UK HE, Chemistry and Theology are two of the subjects you can combine for a Joint Hons degree. Also, Year 1 in Scotland is fairly broad, not specialised, so DS would have a chance to come up to speed in the areas where he is less experienced.
I hope there are other places DS could combine Theology with prep for a laboratory job. Sorry this is so long! But I think his situation is really very complex.
BTW if DS did well in his BTEC and is of a calibre to gain a Theology degree, perhaps he could aspire to more than being a lab assistant? I understand that he may only be able to work part time, in certain environments. He will face challenges. But his educational and career aims can still be rough matches. Best wishes to him.