Fluency/Stammer Issue with 14 Year-Old Boy

I read through the BPN archives but didn't find much about stammering in teens.  I would love to hear from parents about their experiences.  As a toddler, my son stuttered for a few months and then the issue seemed to completely resolve itself for many years.  Then a few months ago a stammer appeared.  (Before, my son struggled sometimes to begin a word, repeating a sound.  Now it is a stammer where he can launch into a word just fine but will repeat the word several times or rephrase his sentence multiple times.)  Oddly, the stammering doesn't seem attached to mood or excitement level.  It comes and goes, sometimes quieting for a couple of days.  Sometimes it even comes and goes within a five-minute conversation.  His teachers have noticed it, but they say he is doing well in school and it does not seem to bother him.  (He is in a small private school with a group of longtime classmates who are usually kind.)  At home it doesn't seem to bother him much either, although lately there have been a few times when he's said "Nevermind" and will get quiet for a few seconds, yet he always tries again and is able to get his thoughts out in the end.  We talked to a speech therapist who said she didn't have a lot of experience with this but that the general rule was not to intervene if the teen is functioning well.  She said the existing therapies don't really work well anyway, especially not without a lot of "buy in" from the teen.  She recommended periodically rephrasing what he's said at the end to reassure him we understood.  She also said to make sure we are listening patiently, and we do try to do that.  We are usually a family that talks about everything openly, but my husband and I have been holding back, not wanting to call attention to the problem in case that adds pressure.  My kid likes school, is social, and participates fully in Scouts and church youth group.  So far, he is fine with reading aloud or speaking in public--even reading verses from the pulpit in church or leading a group of younger Scouts.  We're not a perfect family, but our home life is stable, full of love, and my kid seems happy.  His moods or habits haven't changed in the last few months.  I don't think he's over-scheduled and we have him in a Montessori school that doesn't put pressure on grades or performance levels.  He's not prone to anxiety or depression and has always been on the calm, even-tempered side.  I am good with just taking a wait-and-see attitude, but I want to make sure I'm not missing an opportunity to help.  Also, my son will start a large public high school in August and I worry that this problem will add pressure to the transition.  Any shared thoughts or experiences would be welcome.  Thanks.