Mandy, Time will tell as to whether wifey gets bored and stops accompanying your X and your son on outings. Sometimes they do this to feel like it's a family situation, but in time, they begin to feel/sense that they aren't wanted and will stay away. Let's hope that happens.
I think your X doesn't "think" when it comes to the time he spends w/his son, i.e., wifey in tow. At some point, your son may have to advise his father of how he feels about her tagging along. It's the only way that this situation will get resolved. Right now, the X thinks everything is cool and your son is agreeable to her being along....I would hate to see your son grow up w/anger towards his father always festering in the background and venting later. It's not healthy.
I do hope that things will get better and wifey will find something to do w/her time and disappear when your son is meeting up w/his father.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
