From what I've read I'm pretty sure CBD is the one acting serotonin receptors whereas THC acts primarily on dopamine receptors. I'm by no means an expert though, just someone like you who's interested in trying to ameliorate one's stutter haha Google CBD and 5-HT1a and you should get some results worth reading into. I think hemp oil could definitely be effective, but there's also something called the entourage effect, which is the premise that THC and CBD together have a synergistic effect that's more powerful as a whole than each cannabinoid individually. Just another thing to consider! But yeah I think CBD alone would be beneficial, if anything for any social anxiety and/or stress that comes with stuttering in general.