Magnesium glycinate.

I use the Neviss gummies on Amazon. My 7yo takes a half of one gummy (which is around 50mg of magnesium) and it’s been wonderful for him to wind down.

Magnesium- ask the doctor about dosing, but there are products for kids. It is calming.

OP when does DS wake? Start there and work backwards to create a nightly routine. I would try a small dose of melatonin (like 1 mg) an hour before bed for a few weeks to set the routine. You might need to adjust the dosage up to get the sleep time right. After a good few weeks of routine try tapering it off. If problems arise its time to think about adjusting medications/time taken/ dosage. Changing our after school med to Clonidine is what got us where we wanted to be with sleep.

There's not a lot of research concerning daily long term (years) melatonin use. Tons of research confirming the safety of short term use. Consensus among the Psychiatrists I have asked about it is that the benefits of good sleep to psychiatric health far outweigh theorized side effects of long term use.