If this situation has brought you to the point where you want the relationship to end, then so be it.
You’ll need to consult a solicitor (preferably specialising in family law) to determine if there are grounds of unreasonable behaviour to legally separate &/or divorce. First session can be free & is useful as a stand alone to gain good legal advice so you act from firm principles. Also as children are involved you do need good advice about how to persuade your OH to leave the family home.
BUT if you are not prepared or willing to end the relationship, then you will BOTH need to see his GP together to discuss what has happened with the behavioural changes, drug testing & what to do next. Outcome may be clinic based ultra-reliable drug testing, referral to specialist unit etc.
Your OH’s behaviour may be off for many reasons & the past few months have been challenging enough for us all, let alone those who are being treated for depression & anxiety.
Me, I’d see both the solicitor & insist on GP appointment (in person or online). The very least your OH can do is discuss this relapse with the GP (with you there).
That your OH uses around the children or in the family home is a non-starter.
You need to be on firm ground so that you can take action to live unencumbered by the dead weight of a drug dependent partner.
BTW, addicts lie & lie very well, mostly to themselves so it’s easy to lie to everyone else.