Jimjams, it must be frustrating for you to see that your son would be getting speech and language therapy five times a week if he was in the States. However, indirect therapy can be very effective too, as long as the person carrying out the programme is aware of exactly what they should be doing and why they should be doing it.
As ScummyMummy said, it sounds like your son's nursery are keen to do well by him. Would one of the staff be able to attend the next SALT session with you, or could the SALT come to the nursery? Even if you normally attend a clinic I would think that this should be possible - at least for a one off. Could you video part of the next session so thtat staff have a reminder of exactly what they should be doing? Problems with indirect therapy tend to occur when the SALT explains to x, x explains to y, y explains to z etc and essential information becomes diluted.
Seeing a SALT every six weeks is clearly inadequate and I realise that my suggestions are not the level of SALT that you aspire to, but it should still be beneficial for your son and the responsibility for daily practice would not be solely on you. What is essential though is that as your son makes progress the SALT is able to review the programme and make appropriate changes. If changes do need to be made between appointments could you contact your SALT by phone. I am sure that she would be willing to revise the programme this way.
When the statement is written would it be possible to include your son's 1:1 support attending therapy sessions (and not just SALT?). I remember the statementing process being very political and having to write things like "as jimjams son has no functional speech he would not be able to contribute to classroom discussions" in order to have SALT included under section 3. I'm sure you've already included the fact that the school should have the PECS software available to them.
Have you contacted AFASIC for advice on the best way forward? Their website is www.afasic.org.uk and they have a telephone helpline 0845 3555577.
Have to admit that I've never heard of the Kaufman cards. The Nuffield system was the only one we were taught about when I trained. The Nuffield programme does contain ideas for helping children to know how to position their lips and tongue for the sound they are aiming to produce. As you said your son is a "complex" case in professional terms so I don't know how appropriate they would be for him. Some involve mirror work, which I guess would not be appropriate as it involves copying, but others are more sensory/invasive eg. holding his nose as he says "m" to elicit a "b".
Remember as well that you are also encouraging his speech to develop through your work with PECS as the intial programme found that of the 66 children who used PECS for more than a year 44 then developed independent speech and another 14 used speech augmented by the PECs system, even though traditional therapy methods had not been successful.
The fact that your son can differentiate between sounds is very positive and it sounds like you're doing a great job of supporting his speech and language development. I hope that you manage to get some more support for him soon. hmb's idea of using a student could be very beneficial. If you can find a 4th year or Msc student interested in doing their dissertation on dyspraxia they will have access to the latest research and support from an experienced clinician.
Haven't got time to preview this so hope it all makes sense.
CER