Firstly you have my sympathies. It all seems very easy to say "you have 6 choices, make sure 1 is one you are sure of getting", but I know the horror of realising that actually, there is NO school that falls into that category. Our nearest school is the wrong church, our next 2 are over subscribed and there is no guarantee that we would get in on distance, and the next seems to run a lottery based on a NVR test (so they put the children into bands and take a random number in each). Once we go further then we have to see whether we are churchy enough for the church schools, or ds has to pass a aptitude test etc. In London it seems all too easy not to be able to get into your nearest 6 schools AT ALL!
In terms of last minute prep, IF your son is motivated, then you could certainly do a lot in a couple of months, and I wouldn't view this to be a waste regardless of what school he ends up at (and as I remind ds, her will definitely be going to a school somewhere!). St Dunstan's isn't the most selective of schools, and if level 5s are in sight, then really you just want to cover gaps and practice exam technique. Are you aware that St Dunstans run their own scheme to prepare (based on 3 or 4 Saturday mornings at the end of November)? Another place to look is Old Palace in Croydon. Although it is a girls school they are running an 11+ prep course in half term which might be useful. If nothing else I would expect that by the end of the week you will have an idea as to what he is capable of. If the courses don't appeal then I would recommend using the Bon "How to do 11+ maths, English etc". They seem to be quite good in working out where the weak spots are, so that you can quickly see what areas need to be covered. I'm afraid that I'm a great believer in practice, and I do think that just getting into the habit of answering these types of questions under time pressure leads to an improvement in scores.
If your ds hasn't done any mocks, then hopefully one of the courses mentioned will help. Otherwise you might want to sign up for a random aptitude test, just so that your son has the experience of going to a strange school and sitting an exam with children he hasn't seen before. Shirley High in Croydon has an aptitude test on Sat 9th Nov and you can apply to sit it until 21st October. There may be other schools which are similar but closer to you (I'm just conscious of those dates - many exams have already happened)