Often situations like this can be hard to unpick what is the root cause of the issue.
Use of cannabis and depression often go hand in hand but it's not clear if the drug use is part of blunting the disassociation caused by depression or a causal input into the depression.
In terms of the latter there has been no clinically proven study (though a link between use and psychosis or schizophrenia has been evidenced), but it's well established that there is a relationship between the two.
Even if cannabis is being used to blunt his depression, it's still not helpful because of its side effects that can include increased anxiety and also the fact it might take the edge off just enough not to seek medical assistance to treat the depression more effectively.
In terms of what you can do to help the answer is bugger all.
He's an adult and has to want to make changes himself.
His father isn't helping by enabling poor choices by letting him live rent free, giving him extra money he then fritters and thus free from consequences.
I've seen many people lost in this haze of cannabis and depression, wasting their youth away.
IME some do change, usually when they stop being enabled or tbh when it stops being "fun" being a waster. Hitting 35/40 when all your friends have moved on with life and you're no longer someone they want to hang out and chill with can be a wake up call.