right, don't take this as expert opinion because I don't work with very small children, more with teens. However, we are still on this level of learning with some people.
As far as those I work with goes, trying to teach them separate letters simply doesn't work, because basically it doesn't mean anything - thus they find it confusing. They need whole words. I have found over the years that basically we need to treat our students with asc (autistic spectrum conditions) as if they have dyslexia - which might sound weird, but to an extent there are many parallels. We tend to look at whole words alongside a symbol or photograph, as many people with asc are visual learners. A big part of the autism is that they have limited (or no) ability to imagine something which isn't straight in front of them, thus everything needs to be concrete. Meaning that you show them one of the thing you are talking about. I've had successful programmes where we've made an initial letter out of bendy foam / lego / salt dough! often the salt dough or paint we use has a smell to it as well, going truly multisensory - like putting curry powder in the red paint, lavender in the purple paint or whatever... but on the whole teaching single letters isn't anywhere near the top of the list in our priorities.
Many very able people I've worked with who are capable of explaining their autism in extremely clear terms, tell me that they consider themselves "forgetful" - which of course they are not per se, but they have difficulties with short term memory and become anxious very quickly, having easily accessible lists does help. Every one of the students I work with has their own card with a minimum of "now and next" pictures/symbols/photo's on it; some also have anything up to a whole day worth of activities. They are in pictures or whatever that person uses, and are on a strip of velcro, placed in order from left to right. When each thing is done, the picture (or whatever) is removed - as "finished" is the most important thing for students with autism. They need to know where things begin and end so that they understand what's happening, otherwise the anxiety takes over and they achieve nothing.
Neolara's stuff sounds fantastic. I'm wondering if there is any way to make that approach even more multisensory, as well as a written letter and spoken sound, maybe add tactile elements (some letters in sandpaper, some in different texture of fabric) and even another sensory aspect like smell.
IMHO, he really does need some autism specific input, as the mainstream environment is often very very difficult for children with asc to work in, too much noise, bright lights, distractions and confusion (anyone moving in the room is unpredictable and therefore stressful for them).
sorry, huge post, amd trying to cram tons of info into a short (ish) space. Would suggest that they look at changing the priorities completely, he may need to understand more and attach more meaning to things before he can learn to actually read.