Pot is not making him skinny and looking like death. Heavy pot users become unmotivated or spacey but it doesn't kill people and they don't lose weight. Without a doubt he must be using other drugs, alcohol or coke being most likely. Coke, amphetamines and heroin all cause weight loss. If he's living with a cokehead, that's likely the drug he's abusing.
I advise you to scale back your expenses or increase your salary so that you can survive without child support, as he might easily lose his job if he's that messed up. You can't count on his contribution.
