Advice for communicating with someone with a stutter.

Advice for communicating with someone with a stutter. I have other issues with anxiety while speaking, but not a stutter. I recently began a new job with a cool guy that has a stutter. It is usually only a big problem for him when speaking to supervisors but when we are talking it does occur. There are a couple supervisors that are just plain dicks in my opinion, telling him to slow down and breath, getting impatient waiting for answers, etc. Anyway it seems that this ends up causing more harm than good. I figured reddit would have a sub where I could get some pointers on how to react when he stutters. I just ignore it and pretend I never noticed, but feel that may be patronizing. I often know what he is trying to get at, but don't want to jump in and finish his sentence. Is acting like nothing unusual is going on the wrong thing to do when someone is obviously getting frustrated? I apologize in advance if I chose my words poorly or am saying anything out of line. I have never known anyone with a stutter.