One method of teaching writing to developmentally disabled students is to use a "scribe." The scribe can be a student in their class who does not have developmental delays, a parent, a teacher, or a paraprofessional. A scribe works for students who can communicate orally their ideas but have difficulty physically writing their ideas down.
For example, if you are working on writing a simple paragraph with a topic sentence, three supporting sentences, and a conclusion with a student who is 10 years old and developmentally disabled, he understands what needs to go in the paragraph. He can pick a topic he is interested in, and he can create the sentences in his head. He can speak them to the scribe, and the scribe can write them down. The student can either read the sentences out loud then to revise and edit or the scribe can. The student has "written" a solid paragraph in spite of any developmental delays.
Two modifications of this idea are for students to use a computer if they are able to type but can't hold a pencil OR for students to use a tape recorder for their ideas. Some school districts may be able to afford voice-activated software, which will also work.