Punctuation marks give meaning to what we are reading and writing. Very often, dyslexics omit or ignore punctuation marks because they are symbols, and some symbols can confuse dyslexics if they are not able to connect how the mark looks, to what it's called, how to use it, where to place it in relation to a word and what you should do when you come across the mark when reading (stop, short stop, pause or continue reading). Life would be so confusing without proper punctuation marks!
Once the gaps are addressed, do something fun with your child to reinforce the learning, like getting him/her to spot and highlight as many different punctuation marks as he/she could find in an old magazine, newspaper or book.
P/s: get hold of a good grammar book (with a punctuation mark explanation section) for use when instructing the child.