Thanks all.
He did some revision but in reality very little practical stuff, I think he flicked through the study guides for half an hour.
He is bright but his self confidence is at zero. We did some exam technique stuff with him and he definitely thinks that helped. He's ended up with 2 Bs, 1 C, 2 D's and 2 Fails.
He really wants to do National 5 maths this year as he is really not enjoying it and the thought of another year doing it again is filling him with dread. I think he is motivated enough to study hard. His last report said that he seems to manage well enough in class and understand the technique they are studying at the time but he seems to be unable to translate that into exams. He hasn't had an issue up until the last year or so. My feeling is that he needs to intensively practice questions until he gets to the point where he can recognise what the question is asking him to do/what maths technique he should use. On that basis, i'd be supporting him to do Nat 5 this year and hope the school would do the same, but I can definitely see the benefit to doing it next year too.
We will come up with a proper supervised study schedule this weekend and combine that with raising his confidence and cracking the whip. The studying will be done away from his bedroom and temptation. What we did with DS1 was give him a small netbook with no capacity for gaming and allowed him to use that in the study room for doing things like the bitesize on-line units.
He's accepted that he needs to spend the next few months focussed in order to make his 5th year a better year.
There isn't a great tutor availability locally. I think we are going to get DS1 to spend some time with him showing him how to do the questions. DS1 got 98% in last years Nat 5 and is very patient with him. He did spend an hour with him before the prelim but I think it was just too little too late as DS2 did say he found it useful.
On a brighter note he nearly got an A in Modern and got 64% in his English, only just got a C in History but was one of 7 out of 40 to pass. His BM and Chemistry were both mid 40s, so there is hope really in all of those that he could get a pass/decent pass. His French was mid 30s but he apparently got zero for the written paper but we don't yet know why. tbh I'm not too bothered about French. I've said it would be a shame to waste a year of work though so he should find out where he went wrong in the written paper as he could perhaps get a pass there.
It's Maths that is the issue - he's convinced himself he can't do it and is saying that in general he expects the worse in everything as then he isn't disappointed....that attitude isn't helping. He's also made some comments that he isn't doing that bad compared to the other boys. I've told him he's a pupil not a boy and also to stop comparing himself to others, he needs to instead compare his performance to his ability.
We've managed to not shout but have said we'll be disappointed on his behalf if he doesn't match his results to his ability - no idea if we are doing the right thing or not. 
Sorry for the essay and well done if you managed to read all that!