Tbh, everyone is different and you'll have to try different things to find what works best for your come. What i can recommend is try to find a speech therapist that works for your child. Growing up 90% of every speech therapist i saw did nothing to help me, but i did eventually find the perfect therapist for my needs. It sounds like your child needs a therapist that'll teach him breathing exercises, different types of stuttering, how to identify it before it happens, and also how to identify and control the muscles in the mouth/jaw/neck to work out of a stutter when they happen. In my adult life i have found CBD has offered relief, but i wouldn't recommend it for a child. Most importantly always be an ally. It's incredibly frustrating, most times it feels like you're trapped in your own head. Your stutter has good days and bad days, so just always remind yourself to be patient with your child and be a safe place for them to vent their frustrations. I wholeheartedly wish you guys the best.