Forgive me, but if you found him crying he is clearly stressed, and to me, a clearly stressed child needs the option of 1. A complete break for some time 2. The opportunity to pull out completely 3. Carry on but with some kind of action to make him less stressed.
I know you only have his best interests at heart, but if he is this stressed, is it really worth it? Have you talked to him? Is it something he wants to do?
Please don't feel quilty, you naturally want the best for him, but in this pressure to reach the GS, its easy to lose perspective on what really matters, and what you clearly know, which is for him to be content.
It may be you decide between you that the extra work is worth it (is his comprehension meaning he is well below expected qualifying rate, or just not 'safely' over it?) - if the stretch is a big one, AND he is stressed, if it were me I'd be actively asking him to give it up and to BIG UP the alternative. Is there only 1 school you can try as an alternative? Is it awful really? Are there not others you could try for in other towns?
Further beyond that, and real blue sky ideas, but is moving an option? Is an indie possible? Lots of lovely small indies around that don't charge an arm and a leg. - Advance on the house mortgage - £50k? Not saying any of this is possible, but my overwheleming feeling is that you see your son unhappy but (with the absolute kindest and best will in the world) you are feeling you must carry on as you have no other choice, but it doesn't make him any more likely to qualify if its not right for him.
If he is upset by the pressure now, it is sensible to consider how he will cope in an academically pressurised environment at school.
Just being really honest and I don't mean in any way to suggest what you have done is bad - you clearly care and want the best for him, I just think you might benefit from really facing the alternatives and trying to find a way to spin them better, and also by talking to him.