Quote Quoting Katie
How do I handle this? I don't want my son spending time in a place with unsecured guns lurking around. It is not my intention to come between my ex and his son, but it IS my business whether he is in danger spending time in that house.....right?
A court, in granting your ex- parenting time, has assessed him capable of providing an appropriate environment for the child. Parents often want more details on what an ex- is doing, or how an ex- is living, than the ex- is willing to provide. There's not necessarily a remedy when that happens.

Quote Quoting Katie
And furthermore, HOW ON EARTH do my ex and I co-parent when we can't even have a discussion like this without it turning into an argument?
If this discussion turned into an argument, it is because it isn't occurring in a vacuum, but is weighed down by the baggage of your past relationship, past conflicts, and divorce. You're right that you (that is, you and he) need to find a way to communicate better. If you can't, you may wish to consider some form of parenting time facilitator to assist with communication and coordination issues.