Having grown up with a sibling who has daily tantrums, I feel for your family. It is a heavy burden for all of you, terribly traumatic especially for your S having the tantrums. As the sibling who grew up with this, it influences my behavioral patterns to this day, both positive and negative.
Having watched this play out over the decades, I’d like to offer a counter to some of what you mentioned. As with my family of origin, I’m sure you are trying very hard to do the right thing.
Therapy, no matter the underlying cause, works with the emotional responses to conditions and pathological behavioral patterns. A young person can benefit from learning to work with a professional in a therapeutic context, even if it takes some time to find the right therapist. There are many therapeutic modalities and finding someone versed in the right sort of therapy may be very helpful for the rest of you as well as your S.
Psychiatric meds of course can have side effects but also help many people. Getting the dysfunctional behavioral patterns in check, ie stopping the tantrums, will help you S learn other ways of responding to stress and give him more positive feedback from others. Medications are not a lifelong sentence necessarily, but seeing what is possible with medical management would be another tool to help bring some happier ways of interacting to your family. My sibling was not medicated, though ADD meds were suggested way back when. I remember the peace of a few days when meds were tried, and the peace that descended over our family. We were also the sort of family that avoided meds. Having more positive interactions may help your Ss self esteem, and improved focus through medication might make school a more rewarding environment.
My sibling has been struggling with the self esteem issues from her dysfunctional childhood all her adult life. Meds have made a world of difference. We were just laughing a few days ago about how naming one of her beasts -Aspergers-has brought such understanding that weird is not her “fault”, but a named syndrome to be understood.
Forgive me if this seems like excessive projection. This is a difficult row to hoe. Wishing you the best.