Hello op! So I am 15 and I stutter since I was like 2-3 idk exactly so my advice is be very calm with him talk to him slowly and gently (atleast this works for me) but if nothing works you have to accept that he might never gonna get better (I know it sound bad but reducing how much you stutter is a real challenge) also speaking from personal experience my stutter gets a bit better when I am not thinking about how much I stutter. Also never tell him that slown down bud keep calm for me this sentence freaks me out to the point where I have to a 2 minute break from speaking to calm down. I know I didn’t help much but these kinda worked for me. Good luck for him in the future!