Definitely sounds like he went through some form of vasovagal syncope, which is marked by drop in BP, heart rate and, loss of conciousness (postural hypotension). I suffer that with needles but there is other things that can cause syncope episodes and it could be caused by another underlying medical condition so don't look at its symptoms, look at all the rest. Basically, his illness could be triggering this as another side effect and syncope has dozens of triggers, so be prepared for it, but don't look at just it.

Also look it up so you know exactly what to do if it happens again. In that situation bending him over and putting his head between his knees would have helped restore blood flow to the brain. But there will be warning signs he can feel, lightheadedness, nausea, sweating, ringing in the ears, weakness and visual disturbances such as lights seeming too bright, fuzzy or tunnel vision. So make sure he lies down when he does.