“When my 6-year-old son talks to someone, he will get part of the way through a sentence, then start over again. Is this due to his stimulant medicine, his ADHD, or something else?”
If you don’t remember your son having this problem before he went on the medication, ask his physician to stop the meds temporarily. If the problem goes away, another medication or a lower dose is needed. If the language problem persists, it seems that it was evolving independently of the medication. In this case, he needs to see a speech-language pathologist as soon as possible. [Search for one at the American Speech-Language-Hearing Association]
Don’t delay. Find out what is causing this problem and address it quickly.