1) Those 4 years of street drugs may be a blessing in disguise. SOMETHING worked. And worked well enough to keep him alive for four whole years. His doctors as a teenager didn't, for whatever reason, stumble across 'it' or the right combo to make 'it' for his specific neurochemistry.
2) Every street drug has its medical counterpart.
3) Using this 4 years as a guide, a GOOD neurologist will be able to track the chemicals involved, along with neuro pathways (uptake, reuptake, inhibitors, etc.) and work out the specific mode of action needed IF there isn't a direct parallel (like alcohol or heroin to opioid receptors)
Your son would hand to be completely honest about both the drugs used when he felt BEST as well as other behaviors he may not want to admit to (sex releases both hormones and neurotransmitters, risky behaviors release adrenaline and other chemicals, etc.). Whatever your son was doing that was WORKING on his depression a good neurologist can come up with a short list of variables to put together a treatment plan where he has the RIGHT meds, at the RIGHT dosage (that always takes tweaking) so that he's himself (instead of high), not struggling with addictions, and not having to be in a dark world trying to self medicate himself.
If your son's been suicidal for 10 years.... He's HUGELY strong, and even though he wants to due, is fighting to live. If a neurologist is able to use those 4 years to put together the right meds so he can live with HIMSELF, be himself, and be the man he wants to be? It may not make those years worth it, but it would make the following years worth it, mas oui?
4) After 6 years of doctors and meds that didn't work, your son has learned to have faith that they will NOT work. That there is no option outside of the illegal drug world. It will take even more strength on his part to be willing to TRY, even though the first few attempts may fail again, while trying to get the right combo. On average, it takes about a year. But that year is going to pass no matter what. At the end of that year, he has a chance to be himself again, or be the same & or gone. HONEWORK ASSIGNMENT: Have your son start researching the drugs he used, and their mode of action (hormonal, dopamine reuptake inhibitor, etc. there are dozens), so that when he meets with the neurologist he can be an ACTIVE participant. He's already spent 4 years as a lab rat, so he knows on a personal level what these drugs do. Studying will give him the language. He's not a kid, anymore. That year will pass. How he chooses to spend it will NOT be as a child, but hopefully as an active participant... Who... Due to his trials is PARTICULARLY capable and knowledgeable about how he's affected. He's not a powerless kid. TELL him that. Have him turn ghjbgs on their head and look at how different (knowledge & capability) he is NOW from when he was 'new'. Former addicts make the BEST students of neurology. If he chooses to, he will be able to accomplish amazing things using that HARD won skillset.
Hope some/any of this MIGHT help.
All my best.