Jimjams - I know your son has various difficulties, but hope this is inspiring for you on the language front.
Ds1 started nursery when he was almost 4, and no-one could understand him at all - he spoke in very complex sentences, but had a very limited range of sounds - he couldn't say f, v, s, sh, c, g, p, t, j, ch, z, l, r, ...! i.e. basically he could say b, d, n, m, and that was about it! Now he can say almost everything except s, ch, and j. Also, it hasn't affected his literacy at all - he actually learned to spell out words, to make himself understood! And is now an excellent reader.
He's never been diagnosed, and I've never been given a 'reason', just that 'some children don't pick up the sounds naturally'. But for us once he did learn a sound and worked on it for a month or so, he then quickly assimilated it into his language, and didn't regress back. (Exception being 's' - he didn't have a 's' sound at all, not even a lisp, he would use a completely different sound instead - n, or b, or d. Then he learned the 's', but I think it wasn't corrected quick enough - the joys of monthly SALT, and he started lisping, which we can't seem to shake off.)
Hope the kaufmann cards are good for you - do they suggest an order of sounds to work on? Some are easier than others. Take it very slowly, and just work on one sound at a time.
Anyway, hope you have a good birthday. My other son was 4 last week - can't believe it's gone so fast.