Weird, this drug chemically changes behaviour, so how can his behaviour stay the same?

It's been reported to reduce cravings related to food, alcohol, and cigarettes, you are saying your father while on the drug consumes the same?

Or is it that he uses it for a while, then lapses, repeat ad naseum?

If it's the second scenario, that's personal responsibility, not much to do there really, he has been given all the tools to improve but chooses not to, and that will catch up to him, no matter ozempic usage.