Poor you Scarlet, and poor DS. What a roller coaster 11+ prep is!
I wouldn't worry at all about marks like that at this stage, in fact I would be impressed he managed to almost finish the papers.
I home tutored my DS and DD for the exams in 2010. They are both in Yr 7 at grammar (as, I think your eldest is ?) and although it is quite hard to remember specific grades now for practice tests i do remember that DS in particular was very erratic, particularly as he made so many silly mistakes. I do particularly remember one NVR paper that he had a complete meltdown in - got less than 50%- ended up crying etc. When we got the results of one of his actual tests he finished in the top few who sat it and his best score was NVR
If I were you I would leave the papers for a while and go back to individual questions to take the pressure off you both. You've got plenty of time
I've got 2 more to put through yet so am girding my loins for that. They're both girls though - might be easier!