Whatever the childhood events are, he is doing himself no good by self-medicating & locking himelf away to brood.
He needs professional help.
Cannabis can act as a depressant, so it is ridiculous to attempt to "treat" himself for depression by abusing it.
Smoking every night is going to make him more withdrawn, more paranoid, less confident, less acute, & less motivated.
Depression is an awful thing, but can be managed if the patient is willing. However ... your partner appears unwilling, & instead wants to exacerbate his problems by hiding away with the 'comfort' of his new-found drug of choice. You are clearly concerned about him - but he appears to have no such concern for you.
It is worrying that you feel the need to ask a bunch of strangers on the net whether you would be unreasonable to 'confront' him.
YANBU.
What would be unreasonable is continuing to pour love & concern into a hopeless situation when your partner is refusing to take a single positive step to help himself.
It is not your job to fix him.
He has disapperared up his own fundament 4 times now. What makes you feel that this time, or the next time, or the time after that, are going to be any different?
btw - there is no need to "confront" him.
Simply tell him your position: you love him, & care about him & his MH, but are not prepared to sit by while he destroys himself.
He either accesses specialist MH support for whatever his childhood issues are, & GP help with clinical medication for the depression, or you can no longer bear to watch him treat himself so badly, & are not going to be around to pick up the pieces.
It's Tough Love OP - & you need to adopt it right now, before you too are ground down with the waste & sorrow of it. He needs to help himself, he has no right to expect you to condone his desire to wallow in a misery he is doing nothing to resolve.