Please allow me to offer a few points of information.
Your child may be phonetically inclined, and it seems he may be having trouble separating words from their sounds. He may be also having trouble separating words from other words.
What is his awareness to the situation and is has he been socially affected by it? Does he feel he has a problem or shows frustration? Has it been made light of?
Continue experimenting with conveying verbal instructions in a different manner, with not-too-exaggerated intonations, but clear and crisp, paying particular attention to elocution and stressing the correct syllables. Does he recognize questions in speech, or is he often caught out by the stops that occur when someone is waiting for a response?
In his writing, encourage the correct (and incorrect!) use of punctuation with a special pen, or color. Express and make material the pauses and stops as if they are an integrated part of speech (which they are...)
In his education, show him tally-score with the counting of syllables and the identification of vowels.
Encourage him to read words backwards as well as forwards, and over time, learn to say certain things in 'sdrawckab'.
Work out a game where he can answer whether a word is real or not. Can he ultimately identify nonsensical words and usages in (vocalized) sentences?
Finally, what kind of music does he like? Lyrical promotion and possible rap (a solution to stuttering, by the way!) will lead to clarification.
Can he recite mantras?
(Also, at six years old an introduction to sign-language as a normal thing would guarantee him to stand in good stead in the future, just as simultaneously learning a second language would.
In the end, communal interaction with a collection of children on a day-to-day basis is possibly the most stimulating way for his development to move forward).