Need advice!!

So my fiancé and I have been together for almost 4 years now and ever since I have lived with him his mother has been linked to his bank acoount. My fiancé is 23 almost 24 and his mother pulls out from his account to pay her bills. His family has too many toys that they can't afford and we don't live with them we have our own house and truck payments. My finances bank account had never been under 3k and now he is down to 300$ because the last 6 months she has pulled out close to 2k and hasn't paid any of it back. What do I do? Everytime I tell him that she is stealing his money he says I don't care... I'll wait til I am in the negatives to say soemthing. His father doesn't know the mom does this and e keeps spending and spending.... He won't get a new account and in stressed the F out about money because I relie on him.... HELP!!