Copy and paste this to a file and continue to document all conversations between the 2 of you.
It sounds as though you tried to be helpful.
He sounds like he is still on drugs and not thinking straight. He could also be depressed. Again not your problem, it will be up to him to find the help he needs. Depression makes people unmotivated and unfocused.
You cannot change him, he is who he is.
In the future he will either get his life together and take responsibility for his part of this relationship, or he will not seek help and continue to make excuses and place blame on others.
Always be "age appropriate honest" with your children about their father. Do not include your feelings. "Dad is tired from working." " Dad is not feeling well."
Let them grow up and figure out that mom was a great mom, but dad had his own problems. Resentment will only happen if you bad mouth their addicted dad or he says things about you that they know for a fact is not true..
Some solutions or middle ground? See if ex can call, skype, write letters.. Same on your side.. Offer to have the kids available for calls, skype or let them draw pics and write notes to dad.. 350 miles is a very long distance.. If it were me, I would be there on time to see my children and be very thankful to you for being so gracious letting me stay at your home..
Men are proud, they do not think like women.. All he can think about is how unfair all of this to HIM.. not thinking about the kids first. He is not going to change, he will not listen to you, he is still the teenager he was when he started using drugs and drinking.. Do not waste your time and energy on him.. Use all of your energy on yourself and your children.
I am sending you strength and patience.