Co-worker who stutters question

Co-worker who stutters question This may have been asked before and if this isn't the right place to ask, I apologize. I work with somebody who has a pretty pronounced stutter. Is it ok to finish the word(s) he's stuck on for him on or should I just wait? I don't want to offend him by finishing a word for him nor do I want to make him uncomfortable as I just sit there waiting.