I think the liquid you found and the conversation are 2 different things.

I can't think of anything that accounts for the conversation apart from heroin. There may be a modern street drug that's taken in a similar way though. The way you describe him in the bathroom would fit heroin too.

Talking of syringes isn't that weird. You can use syringes to extract things. But a needle? You would only use a needle if you were injecting something.

No idea what the liquid is. I guess it could be a cannabis refill. If he is playing around with heroin, it could be liquid heroin, which is drinkable.

We really can only guess but you need to have a serious talk about this. He shouldn't be getting to the point he's slumped in the bathroom around your kids anyway, whatever he's taking.