Wow. As a mother of two, I couldn't imagine being away from my kids like that. I hate it when my oldest is gone two days a week once or twice a month (on paper her dad has every weekend but doesn't take every weekend). I feel anxious and can't wait for her to come home. and even when I was using drugs I still made sure my daughter (only had one child when I was using) still had birthday and Christmas gifts.
I feel like it takes a real piece of **** to have kids and abandon them, no offense.
Proud of you that you are stepping up and being the parent your son's need you to be. You aren't at all the bad guy. You have been beyond understanding and nice before this point. I think going to court and having a judge sort the whole thing out is an exelent idea. Who knows, she might even be required to pay you lol.
Hang in there.