It's pretty important to be able to draw confidently, otherwise you will struggle with an architecture degree as there is a lot of communication / analysis of your designs through sketching.
That said - if you are taking DT seriously, and generally enjoy drawing, I would think you should be confident at that kind of drawing anyway - from basic ideas sketches & diagrams, through to rendering illustrations of what a product might look like.
So your DS may be fine with his drawing / art skills if he doesn't do art - most if not all Universities will want to see a portfolio of artwork, and for the student to be able to discuss it - and this may even be more important than whether the candiatate actually has an Art GCSE. But you always phone some admissions offices if you wanted to double check if not having art would be a problem.
I gather (from reading threads on here - back in my day Art O level was really just a 3 hour drawing exam) that Art GCSE is very intensive & covers lots of Art History etc. So if your DS wants to go down that route, it doesn't seem less academic than a humanity, Ebacc or not.
Could you find out whether your DS could do Art at A-level without having done the GCSE? In my day that would have been possible (if you could demonstrate ability) as the O-level was 'just' drawing (more or less) and the A-level was where you put the real work in, but I'm not sure it would work these days.
One thing you may find is that because architecture is such a broad subject - science, art, physics, humanities, maths - no-one can really be expected to have learnt about everything it might cover, certainly not to A-level standard unless they did about 10 of them 
, and the most important thing is to be open-minded & open to learning new things, which it sounds like your DS is.
p.s. an Architect Without Humanity sounds very wrong, like they might design some dreadfully unpleasant buildings, but of course your DS can have Humanity without actually having a GCSE in it!