Bit of a long one

Need a little advice. Found out my partner was cheating a couple of months ago, we are trying to work through things (though that's not going so well). Now I have good reason to believe he's taking drugs. He never has any money despite doing lots of hours at work and I pay for most things. he's been acting weird, going out at all hours and hiding in the toilet. I don't have proof about the drugs thing so if I confront him he will just lie. He lied when I confronted him about cheating but I had proof. He said he would work on our relationship but he's been hiding his phone, flicking off whatever hes looking at when I walk by. Not spending any time together at all. Spends no time with the kids. He doesn't even know what year in school the kids are he pays so little attention.He just will not talk to me at all. Another problem is that one of his family members have just found out they're seriously ill so I don't want to be a jerk and break up with him during this. If I find out he is definitely using drugs then maybe his mum would be able to get through to him but shes seriously stressed and attending hospital appointments ect with the family member. Am just stuck plodding along at the minute not sure what to do....