This advice-column-style blog for SLPs was authored by Pam Marshalla from 2006 to 2015, the archives of which can be explored here. Use the extensive keywords list found in the right-hand column (on mobile: at the bottom of the page) to browse specific topics, or use the search feature to locate specific words or phrases throughout the entire blog.
Q: My son just turned 5. He has been having difficulties with his articulation for several years especially with D, T, TH, S, Z, N and H. also he has difficulties in putting letters together such as D-O-G. He is able say things correctly only when we go really slow and draw out each sound. He also sucks in when trying to make the S sound. How can my husband and I help him with this?
The severity of what you describe requires the assistance of a professional Speech-Language Pathologist. I hope he is seeing one at school or privately.
There is very little I can say about a serious problem like this to a parent in this format, other than to give general advice about speaking clearly to him, reading to him, helping him practice sounds and words, and so forth. Your son is going to require a long-term of therapy at the hands of a professional. That person will help guide you through this; please see someone!