Surely someone must have seen this coming? Has his written work suddenly deteriorated, or have these aspects of essays - capital letters, punctuation, not checking his own work before submitting it - been pointed out before, and been ignored by the child?
Have parents not LOOKED at his written work during the past four years? Punctuation and 'proof reading' should start to be taught in Yr2, if not before.
Are the mistakes just that he forgets, or careless slips, or does he not understand the mechanics of punctuation? By Yr6, in addition to using good words and 'style', students should be using paragraphs and advanced items of punctuation in a creative way, to enhance their written work.
Any of the books for children by Lynne Truss will be useful in reinforcing punctuation, and I give a link below. She is a very entertaining writer, and her books are enjoyable. But also, when he is reading ANY book, encourage him to look at how sentences are constructed and how punctuation is used.
A tutor shouldn't really be necessary, as once he is aware of the problem DS should be able to correct much of it for himself - provided he is prepared to make the effort.
www.amazon.co.uk/Lynne-Truss/e/B000APRCCY
On a lighter note, you might all like to try this: