Story its ok to get angry at your situation and your ex, for its all part of the grieving process. i get that way with my exH for i too am sick of always trying to do the right thing, and be the strong one, and be the one there for my kids, while he and his gf enjoy their life knowing that i will pick up the tab to make sure my kids have the most amazing opportunities i can afford (and yes he can afford them as he earns more than i do, but i don't have a leech living off me). i am the bad cop in the family, but i know at the end of the day, it will be me who they come to for support and advise, and it will be my lessons that they will remember.
the only advise i can offer is be honest with your son, without adding your feelings about your ex's behaviour. your ex does love him, but the addiction has him by his short and curlys, and so he is just shell of the man he was. as Still Learning put it, would you swap places with him right now?