Should I hang out with my ex's mom?

Answer 1

Your ex, is your ex. You need not have any sort of relationship with your ex other than a former relationship.

It was important to say that for a reason: your ex is not necessarily involved in your relationship with his mom. If you have developed a friendship with his mom, that is between you and she. If he tries to involve himself in that, just remind him that he is your ex.

Now, that all being said, relationships are complex things. While it is possible for some people to separate or compartmentalize their relationships, it isn't always, or often, a good thing. Consider all of the consequences before you jump here, and engage perhaps in a brief "cost analysis:" Ask yourself, "Is it worth it?" Be honest, and you'll be happy with your answer.