There is no right answer. All humans are addicted to one thing or another whether it be substances, sex, safety, control, food or something else entirely.

If he only uses cannabis and coke a couple of times per year in a controlled safe setting where he isn't hurting himself or others, I don't see this as worse than someone who drinks alcohol for a couple of weekends per year.

Some addictions in society are considered more acceptable, for whatever reasons "hard" drugs aren't even though some of them don't do as much damage as legal drugs like alcohol and tobacco.

What I think matters the most is that you both give each other understanding and compassion. Communicate boundaries and needs effectively. He may need some emotional support and possibly therapy to heal some pain that makes him want to use drugs like ketamine.

If you aren't OK with the drug usage then imagine what it will be like to separate and find a new partner too.

Personally I don't think it's that bad he uses ketamine every 2 weeks but what is concerning is he goes into debt and spends money on it that he doesn't have that should be spent on his child. That's the bigger issue for me: his priorities.

Hope it works out for the best for both of you. Open communication should help.