Help with RSD child.

My 8year old has just been diagnosed with ASD and ADHD (I always knew he was), my husband and I have always struggled with helping our son to regulate his emotions, his meltdowns can be quite strong. We've tried yoga, breathing, fidget toys, dens to hid in, quiet spaces to go to, pillows to punch but he doesn't turn to this things, he just screams, shouts, threatens violence, breaks things and then feels either awful afterwards or as if nothing ever happened. We need some advice please!