You aren’t an asshole. I (23m) stutter really badly and Stuttering is something that is not understood fully by people who don’t stutter. So you are valid in trying to comfort him. He most likely took your advice as you were being kinda condescending towards his struggles. I only say that bc my gf has done the same thing. You as a non stutterer won’t understand what it’s like to know exactly what you want to say, but as soon as you get to a certain phoneme you block, prolong the sound, or repeat it uncontrollably. I currently see a speech therapist, I would totally recommend you suggest to your boyfriend he also speak with a speech therapist.