I remember your dilemma over sixth form choice from that year's thread, Third because my DS2 went the other way and has done a level 3 BTEC. TBH, it hasn't really suited him because he's so apathetic about working and you need to keep up a constant (but fairly undemanding) level of work. Two of my friends DC who both have SN have thrived at BTECs, though and are on course for DDD or above. One is heading to Portsmouth Uni to do engineering in Sept.
I think you made the right choice for your DS at the time, as he was doing well with his GCSEs and could have achieved high grades in A levels which would have given him more choice of courses at university. With slightly lower A level grades, he's probably no worse off than having done a BTEC route. I don't know the reasons why your DS is not expecting stellar results, but the step up to A levels can be hard in lots of ways for many students, either disliking their subject choice, the sixth form environment or just finding the pressure of work too much. The fact that he's stuck it out is to his credit.
I know I made the right choice for DS2 because he would not have coped with A levels at all. At least he'll come out of college with some sort of qualification and TBH, he's much too behind in his development to start work, so keeping him in education, even with low results, is right for him. He has an unconditional offer to do an HNC next year, but at our local FE college which does a few HE courses. So not even an ex college of HE! I don't care. 