I’ve been stuttering since 8yrs due to a mild stroke I’m 24 now but the thing I’ve found most helpful (I’ve tried all the things you’ve mentioned as well but it’s escapism in a sense) is easing the tensing during a block. So let’s say I’m stammering a subtle voluntary “blah” or exertion to take your mind from the stutter has helped it’s silly at times but it’s better to be silly and say exactly what you want than to give the run around unless if you’re pressed for time the runaround helps but if you’re just chatting do not avoid what you want to say build the motor skills through repetitive trying that way you kind of figure out how much breath you need and other advocates to get the words. Easier said then done especially mentally but just encourage him to finish what he’s trying to say