NotInTheMood
There may be multiple issues, but you do need to look beyond the observable symptoms.
Dyslexia is about having problems with reading writing an spelling, or having problems with a man made communication system the visual notation of speech, decoding and recoding the graphic symbols society chooses to represent the sounds of speech. There are three cognitive subtypes of dyslexia, reading disability, auditory, visual, and attentional. which means that an auditory processing disorder (listening disability), visual processing disorder, an attention disorder, or any combination of the three can cause the dyslexic symptom.
Goping back to how we learn to speak, we learn to speak by listening to, processing the speech of others parents, siblings, peers, etc, and by imitation anbd understanding reprocessing those sounds to produce our own speech. This can be made difficult if you are born with a listening disability, or auditory processing disorder (APD). Which means that potentially an auditory processing disorder could have been a cause or contributory cause to your DSs speech and language delay.
Those who have APD also tend to have very poor sequencing skills, at my APD assessment aged 49 i found out I had the sequencing skills of a typical 4 yr old. The same mechanism in the brain controls our auditory and sequencing abilities.
Those who have APD tend to develop visual compensating abilities to cope wiht our auditory processing deficit, and with regard to the visual notation of speech, text, we match the whole sound of a word to the whole graphic representation of the word, best used in a logographic writing system. Unfortunately our culture uses an alphabet writing system, so we can only match the whole sound of a word the multiple graphic symbols, letters, used to represent the sound of the word. So we match the whole sound to whole graphic representation, which is the letters that make up a word, but due to our poor sequencing skills we can sometimes get the letters muddled up. As you described above. Currently my problem word is "what", which you may see me post as "waht". When i immediately read what i have typed back to myself it all seems OK, it is only later that i see the spelling mistakes and missed words.
Do may be you could be looking at auditory processing disorder (APD) and dyspraxia
you could have a look at the APDUK web site